Wait - essentially you can't and won't say anything about his horrific parenting because you want to marry him? Please step back and take another look at that idea.

This child is not going away any time soon. She will turn from a 13 yo nightmare into a 23 yo nightmare into a 33 yo nightmare etc.  Your SO will continue to enable her, coddle her, and refuse to give you the right to speak your mind in your own home.

This man is not a catch. I know you want it to work for the sake of your child but if you think it's bad now, it's not going to feel any better 5 years from now, when you are even more resentful.

Speak your mind. Tell him that you are unable to tolerate her behavior and his refusal to address it, and so while you will be civil to her, you will also distance yourself as much as possible. If he can't deal with you having an opinion about what goes on in your own home, this is not a relationship you want to hang on to anyway.

relationships with her.  She already has a mother, your are not that person.  BUT she should respect you and your home and family.  You have to disengage somewhat,  What she does with hare farther at your home is there thing. He can cook, Do the wash, take care of her.  You have to take a step back and try to find how you all fit together 

So let me get this right...

"I need tactical information on how to conceal my intense disgust with her & with how he parents her...(I would like a ring & a wedding), but if I can’t do better with this monster of an SD, that won’t happen."

So you just have to fake it long enough to suck that ring out of him, right?

Are you crazy? Why would you hide from him "intense disgust" you feel for his daughter? And the fact that you blame him for her being the way she is?

Sounds as if you're all going to act like there's no problem until you've roped in your man and then you will let loose. That's great planning. When you pick your china pattern you might as well pick a divorce attorney too.

If you don't like the taste of shit sandwiches, why would you marry the chef? How could you think you're ready for marriage when you can't stand the presence of half the package you get with him? What you're suggesting is that you want him to think you like SD long enough to marry him, but then what? You're going to keep biting your tongue for the rest of your marriage?

No. Of course you won't keep biting your tongue. You'll be indignant and resentful by how he treats you and how SD treats you and yet here you are, complaining about it and longing to marry him at the same time. Sounds so silly when I put it like that, right?

You're lying to yourself and to him by biting your tongue and acting as if SD doesn't bother you. You are selling this guy a product you've misrepresented -- a relationship with a woman who can't stand his kid but refuses to "go there" with him to get to the bottom of it -- I guess because it might rock the boat and you might not get your man.

You're going to go through hell with this kid and you're going to put your own kid and him through it too. Be honest and tell him the truth before you marry him. It's the only way you can make the marriage work long term.  

I get it -- you have no leverage. You're already shacking up. You already have a kid with him. The time for negotiating SD's bad behavior was before you bought into all this. And if you're as desperate as you sound, you will put up with anything to get your man, so I'm not sure what advice can help with your question. But if you want to give this marriage the best chance, you can't let something this major go, even if you become master at biting your tongue.

I can't imagine why you'd want to marry this mess...except that you got everything but a ring and that probably stings. Forget your pride about that and think about why you'd even want to be with someone who sounds not only flawed, but completey out of touch with how flawed he is.

Regardless, you can hide the truth from him, but you're only hurting yourself by doing that. You will likely get your man, but you won't want to keep him if you haven't dealt with the hard work of dealing with his kid.

Rather than  learning to lie you to your SO and his toxic prior relationship crotch dropping you would be better served to investigate why you chose to saddle your own child with that shallow and polluted  gene pool and how to mitigate the consequences of you choice for your own child.

This man has created this toxic little shit.  What makes you think he will change how he parents your joint child?

Past behavior is the best indicator of future performance.  You will have to be on 24 hour watch until your own child(ren) launch at 18 if you want to avoid a similar outcome to that of your SD.

Good luck.

At this point, I need tactical information on how to conceal my intense disgust with her & with how he parents her. 

You don’t conceal it. If you’re adamant about staying with this daddy dearest, then you just let it rip. If it’s meant to be, then he’ll relate to your disgust regarding HIS child,  and work to change HIS poor parenting, and align with you accordingly. 

But, he’s showing you he is not okay with it. Isn’t he? 

This kind of cohesive game plan is lost on a lot of us here. This simply isn’t doable. 

A relationship built on a lie is not going to last.  You shouldn't pretend to like his daughter in order to get a ring and marry this man.  If you can treat her with kindness and respect even while detesting her behavior and not liking her, that should be enough.  If it's not, this is not the relationship for you.  If you can't manage to treat her with kindness and respect, the relationship will not work.  

Even assuming you can figure out how to pretend to like his daughter, is that something you'll be able to sustain?  She's 13.  The teen years are just beginning.  It's going to get worse, not better.  And if your BF hasn't figured out how to parent by now, it's unlikely he's going to figure it out at all.

If the daughter is that bad, and your BF isn't willing to make changes in the way he parents her, I've got to think you're better off without the two of them.

I can understand your discontent with your partner, your being stuck in this because you had a child with him, your nightmare of a skid, and your trying to grin and bear it attitude.

But, what is marriage with this man going to get you? Did he tell you that he wants you to "do better" with his daughter or is that what you think you need to do to get him to marry you? Is a divorce payout worth it? Is this what you want and why are you doing it?

I'll admit that I have fantasies of SS14 going to military school, living with his mother full-time, or just disappearing. I always imagine a life without him. But, the truth is that he will always be around and he will always be annoying (personalities don't typically change).

And, my DH knows that I don't like SS. He may not know the extent or he may be blocking it out because he does not want to know, but he knows. He knows that I'm not fond of his parenting style, etc. But unlike you, my DH was the one pushing marriage, so that I would be stuck with him.

Anyway, to answer your question, some ways I've dealt with the SS that I can't stand is (please note, I did some of this to keep my sanity in a house of chaos, so some of these suggestions may sound bizarre, but I was desperate):

1) I used to think of him as a cartoon character or stick figure, so that I wouldn't picture his actual face and get emotionally consumed. So, yeah, if I thought about him or even interacted with him, my mind would think of a cartoon face and body than his. I no longer have to do this for sanity sake.

2) If your SD is indeed cruel, then you don't need to do anything to make her look bad. My SS does it to himself. I am a nice person and everyone we know knows it. SS merely has to spend some time with people to say something dumb, mean, or selfish. He can try to hide it, but it doesn't work for long.

3) Avoid her. And, when/if your DH notices and asks why, have an honest conversation with him about her behavior. If he has agreed to parent your child together differently, then he must get that he effed up his daughter. My DH regularly says that we parent ours differently and he wants to do a better job this time. He gets it (but follow-through is a completely other story). 

4) Over the years, I have learned to block words out. It was hard for me, but I try not to listen to words that come out of SS's mouth. I sometimes spend more time than I should in the bathroom or bedroom. I use those places to just breathe when things are getting out of control. Sometimes (before DS was born) I'd just walk away and take a long shower to decompress.

5) Early in the relationship. I tried to do the "fake it til you make it". This was DH's suggestion too. He knew I wasn't bonding with SS. Well, I tried it, but SS's actions couldn't allow me to "make it". He was too much of a mess. I used to hug him, tell him that I loved him when saying goodbye, stuff that I would easily say to his sister and mean it. But, it was eating at me, so eventually I stopped and it feels much more authentic. I know DH hopes that I'll love him, but no, that will ever happen.

6) I used to go on vacations with skids, celebrate birthdays out with skids, take the skids out to nice restaurants, but I stopped doing a lot of it. I've explained to DH why and he gets it. If he wants to do those things, he can do them himself with them. If he wants me to join, he has to compromise (I got sick of watching him (and me) fund expensive stuff).

7) Tapping Therapy - look it up. My therapist told me about this and it was useful when I was in stressful situations with the skids that I thought I would just start yelling at everyone or crying. I'd go to the bathroom and tap. It would calm me down immediately so I could get through whatever was going on.

I acquieced to a lot of things early in my relationship that made me hate myself and despise the skids. I compromised on my beliefs, wants, etc. It was only when I was more honest with my DH and honest with what I could take, that I felt better. I don't know if it would have spelled the end of my relationship if I could have been honest earlier, but I do know that I suffered a lot and I still harbor a lot of resentment from those early years, and that doesn't even include what I currently deal with.

Don't pretend, be honest with SO and disengage. When DH asks about SD10 I am totally honest with him. 

For example he has said something like "Try to include SD in _____" My response "If it's not the iPad, she's not interested." etc etc. And leave it at that. If he brings up her behavior or your interactions with her, just be honest. 

I wouldn take marriage off the table. Why complicate an already difficult situation. Trust me, you may think marriage will bring some level of stability to your life, but it won't. Instead you will feel more trapped than you do now.

As far as SD you don't have to like her. You don't have to pretend. All you have to do is be an adult and be civil, no different than when you have to share a workspace with someone you don't like. 

I will never like my SD, I will never trust my SD. That ship sailed.  When she is here unless she approaches me I just go about my business like she isn't even here. The great thing about that is she is the type that's wants you to chase her and kiss her rear end. SO has trained her that if she hides in her room or is moody and sulking he will coddle her. This works to my benefit because I can usually go all week without seeing or speaking to her because she won't make the effort to interact with me. 

I don't make plans to do anything with her and I don't buy her things. That's all SOs responsibility. I don't pay any attention or concern myself in any way with her behavior. Again that's his problem. You will find in time as your behavior changes so does your feelings and attitudes towards SKs. I don't feel anything good or bad towards SD anymore. Nothing she does really bothers me and I don't care about how stressed or upset SO becomes with her because that's his problem, not mine.