Well, disengaging takes a lot of forms. You can still do certain things if you want to, and let other things go. For example, if it would help YOUR vacation (ie less stress) if you packed her bag, then do it.  If it's easy enough to pick up stuff she needs at the store (I assume she's out of diapers!), then do it.

Then you can stay out of other facets of parenting that might not affect you as much in an emotional sense. So maybe you don't transport her between homes anymore, or make sure she bathes or eats her vegetables, or whatever you feel you do too much of that you can let go of.

Partly though, for you - I'm guessing disengaging feels wrong because your partner doesn't parent adequately if you don't pick up the slack, so you feel you are contributing to your SD's neglect (I mean really, he couldn't run to the store for diapers; or give SD a bath and put her to bed in just her undies when the pajamas were forgotten?) if you don't help.  That's a whole other thing to sort out - whether you can feel okay living in a situation where a child is getting poor care, or not. And if you can still respect your DH.

On the other hand, if you stop overfunctioning as a parent, he might stop underfunctioning and do better (though honestly, it sounds like he's lazy in that area).

If you don't step back, how will your dh ever learn to cope? The more you "help", the harder it is for him to progress. 

I was able to disengage as much as I wanted to because my DH was not a crap father. Had he been a crap father, I don't think we would have survived.

Disengaging can be a tool of survival IF it works to lessen your anxiety and it doesn't seem to be doing that, in your case. Sure, you can go back to doing the parenting but as Winterglow stated, how will your SO step up if you step in? And, if you step back in, what about the resentment that you will feel for having to do so?

If disengaging is right, why does it feel so wrong?

It's not right for everybody. You have to find the balance that works for you. You have to find the level of disengagement that works for you and your relationship. It's trial and error.

You have stated that your criticism of his parenting causes arguments, can you have a discussion (without anybody getting defensive) about your expectations, things that each of you can compromise on, a plan to move forward? He is her parent and I think he needs to commit to parenting her, for her sake, as well as the sake of the relationship.

I used to feel the same way especially when it came to things that I felt were basic needs.

Your SKs are young so your feelings are understandable. But is DH doesn't learn to parent now Thier will be much worse to come.

Also you can read so many SPs in here dedicated themselves to being the best SP to SKs they could only to be completely unappreciated and disregarded later on. 

That is a much more painful experience than disengaging.

If your partner asked you to pick up diapers or said 'shall I put her to bed in undies or in her clothes?' I think it would be fine to 'help' but if he doesn't ask for your assistance you have up assume he's fine with his parenting decisions. It's so hard, so you have my sympathies. My ss and sd keep losing their library books. If they ask me to help look I will. But if the library says they can't get any more books and they're really disappointed I will not go and look for them or pay their fines or get them books on my account - that's their dad's job. 

If you are there to support your partner when he needs you to you are doing enough. If things bother you about his daughter it's probably due to his parenting and you need to bring it up with him. Only he can change it. Often there are oversights and different priorities and they can be resolved with some work.

I would also say don't  worry about what disengaging is supposed to look like. It doesn't mean you can't enjoy your skids or give them treats or teach them things or ask for meaningful change to make your life easier and happier. It means you won't continue to pull your hair out over things you can't control! 

Make different choices on how you feel.  

You can disengage and still influence.  Instead of doing these things for SD and DH, give DH a packing list. All it takes is for you to type it up once in Word and then print it out and hand it to DH before each trip with SD.  If he fails to pack to the list, that is on him.

... because we feel it's right to be polite and helpful. We want to fix things and contribute and be part of whatever group we belong to. Disengaging feels contrary.

I've done quite a few such "contrary" things in my life, and none of them felt good or right at the time. In retrospect, I'm very glad that I did most of those things, and only regret not having done more of them.

GIve proper respect and latitude to your bad self. You'll find she's right more often than you imagine.

Question you need to ask yourself: why was it so appealing to stay with a man with a drinking problem who neglected his child? What made you think "this is the man I'm going to hitch my wagon to"?

The problem here isn't wholly your "man". It's your lack of self esteem or your need for a fixer-upper project. Disengaging has an abundance of uses, and here it needs to be used to open your eyes to the type of man you're involved with. Why does this behavior irritate you? Because it reflects poorly on you. It reflects poorly on your training of him, or on your ability to choose a good husband, or insert whatever else it makes you feel.

You disengaged and he didn't step up. That means you've overfunctioned for him. It's not that this comes "naturally" to you so you should do it. He should learn. He isn't learning (or he did learn, but your standards are way too high for him to meet). This discomfort you feel is pointing out weak spots in your relationship that you think you can fix if you just step in.

Girl, no. Forgotten pajamas might be annoying, but they shouldn't cause you to throw out disengagement. Either your standards are too high, which means you need disengagement to chill TF out, OR he continues to fail as a parent with the pajamas just being the latest issue in a line of many, whichs means you need to decide if you really want to stay with a man who is neglectful of his daughter.

What you WANT to do and what you SHOULD do don't align here. You WANT to come in and save the day, but that's not healthy or fair to yourself. You CANNOT save someone from their own life decisions, so stop trying. I know this sounds incredibly harsh, but what you end up doing is causing resentment, even if silently. 

You have to watch the man your SO is, not the man he can be when you do the heavy lifting for him. If he isn't the man you think he should be, then you can express that to him so he can improve. If he won't do that, then you have to decide if he's the man for you.

Start teaching SD how to be independent. I have done this with SS12 - I started around the time he was 8 because I was seeing that if I didn't do it it didn't get done. It has helped with disengaging. For a trip I will give him a list of things he needs to bring its up to him to pack it. He does his own dishes and laundry - my theory is if they can work an iPad or X-Box they can work a washing machine. He has a set of daily and weekly chores that are age appropriate. DH said he never had to do chores growing up and it shows. 

Now that SS has started becoming a crappy teen  he is conditioned, at least, to take care of himself in the most basic manner. He knows he is expected to clean up after himself, where clean clothes, keep his room and bathroom clean, and keep his grades up. He knows what manners are expected when he is with me. 

Now when he is with DH or BM it gets a little crazy. DH has started to comment that when I am home SS behaves better. NO! He just knows my expectations. When DH is around I don't do anything for SS. I try not to take on any parenting role, but I have certain expectations for anyone living in my house. And I use the Choice - Consequence method. Good choices = good consequences. Bad choices = Bad consequeses. But these rules apply to everyone. Parenting is left to DH and BM and that part isn't going so well.