Is it possible for you to distance yourself from him? I know it's best for stepparents to be involved, especially when there are other kids involved - you want everyone to be a family and act like siblings/parents/etc. but sometimes it simply doesn't work that way, especially when one parent looks past their own child's bad behaviour and you're left trying to resolve issues that weren't created by you to begin with.

I think if I were you, I'd step back, ask my DH to start sorting out the annoying, misbehaved child, and I'd focus on my own kids. You're resentful because so much of his bad habits have been put on you to resolve because otherwise you and your kids had to deal with it. That's not fair. DH should have stepped in and fixed this but you did, or tried to, and now you're resentful. That's normal and natural, but you should step back.

My SS11 was really really horrible when he was younger and my DH does not discipline his kids. It got to the point that I refused to be in public with SS and just hearing the sound of his voice would piss me straight off.

For me it really just took time. Time for the memories of bratty little boy SS to fade and be replaced by other things. He's older now so he does understand better. So when he is annoying the heck out of me I just flat out tell him that he is on thin ice and I'm about to get seriously pissed off. More often than not, that actually works.

Like most things, the memories of his early childhood will fade and become less concrete. Hopefully his change in behavior will continue until he is just a normal little boy, with the annoying and the awesome parts that go along with childhood.

From personal experience, the only way I got past it was over the passage of time. I've been with dh nearly 11 years, married for nearly 8, and he got custody of his 3 boys when they were 3, 4 & 9. Their behaviour was absolutely appalling - violent tantrums that went on for hours, constant whining, crying and attention seeking, food issues, constant bullying/fighting each other way beyond normal sibling squabbling and it was a complete nightmare! I felt so resentful towards them, their bm and my dh for not stopping bm's emotional abuse of the boys which was the root cause of their bad behaviour, though I realise now it wasn't as simple as that as she was abusing and controlling him too and saving the worst of her abuse towards the kids for when dh was away working. I seriously questioned whether to continue in the relationship with the love of my life because of his kids, and if he hadn't been a great dad who got the situation under control I would not have stayed with him. Through very strict rules, firm boundaries and clear consequences that were always followed through without exception, along with daily routines so they knew what to expect and when, in a loving and secure home with two calm adults showing a united front, the boys are now unrecognisable as the little savages they once were. They've grown into great kids, the oldest is now 20 and doing well, the younger ones are 13 and 14 and well-behaved, respectful teens. They still have contact with bm and ss20 tells us they still misbehave for her, and ss13 especially back talks her and doesn't listen, but they have no problem with us, school or other adults so that's all down to bm's shit parenting.
I think for me the resentment decreased over time as I saw an improvement in their behaviour, and knowing that my dh was stepping up and doing what needed to be done. Knowing why they were like they were and realising they were simply acting out because they were innocent victims of bm's abuse helped too (hence why dh got custody). We did have all that confirmed by child mental health services who gave us guidance in how to support them and address the behaviour which was really helpful too. As for holding onto resentment from actual things that they did, I let that go a long time ago, again through understanding why it happened and seeing the massive improvements they've made, though I imagine if I'd had a child younger than them or of a similar age who'd been on the receiving end of their step-siblings bullying and bad behaviour I might have found that harder to deal with so I can understand why you're struggling.

It can be a vicious cycle where their bad behavior causes our resentment and then we distance ourselves or over react etc and then the child associates us with getting into trouble and acts up more so we get more distracted and resentful. Breaking the cycle and trying to see the child beneath the behavior can be tricky. We have to recognize though that we are the adults and we need to change our own actions to help them to change theirs back towards us. So that could be disengagement or it could be just trying to praise more than we criticize or trying new parenting schemes to encourage good behavior. I think that there is a lot of faking it until you make it involved. Trying to ignore and not act in our resentments so that in time a better relationship can be established and the resentments will fade. It is not easy but if we want to make a positive difference then we need to start with ourselves.

I would also have a frank discussion with your husband and say that sometimes you find yourself placing blame on ss when he missbehaves when really he is just acting how any child would given the circumstances and the blame really lies with the adults who set up the circumstances. That you would like to improve your relationship with ss but that you feel that things at home need to change and you want to start that and you need your husband to help by doing abc and enforcing xyz rules.

You just opened a closed box in my closet... I grew up with 3 brothers from hell !!!

boys are just gross lol, and annoying, and irritates the crap out of you - I get how DD feels.... I will teach her the following, SS pulling her hair, pull his back... just a bit harder, SS grabbing her toys, take it back with force...

Yes I know people do not agree with my methods, but my parents never got involved when the 4 of us argues or fought lol, My mum said - let the children sort themselves out.... just like adults should not involve their children in arguments. The older we got the less fights we had and now we are all friends and we can laugh about everything.
It's part of sibling bonding.... Simply tell DD to deal with it in her own way, and if SS has a little temper tantrum all you and DH do is laugh and say, well stop bugging DD and she will not slap the shyt out of you...

You only supervise not saying anything.. to ensure that they do not kill each other, if one of them should pick up a knife, then both get punished...before they kill each other.. you will see that they start having less and less fights, cause no one is paying attention (or this is what they will think)

Then if they are naughty together... sometime you pretend not to notice and sometimes you punish both, oh as children.. we plotted revenge on those punished together sessions... my parents made us all 4 sit in one room and then work on an apology and strategy how to redeem the trust... I'm not going to reveal what was talked about behind that closed door, you can only imagine 4 kids plotting revenge on evil parents who do not love them...

When the 2 kids stand together, you will automatically forget about the bad things of the past, cause you have the power now to cause a sibling relationship between to children and it will be nice to see, for years to come, just put the past behind you, let the kids sort each other out and 20 years from now, around a bbq fire with new spouses and grand babies, these stories will be told and all will laugh about it