Fell, Fafoutis and Shirley,
I've steered clear of "step-parenting" so far, but this thread showed up in active... The way I see it, the DC experience belongs in the wider discussion on step-parenting... It's as much a part of the issue, surely? But I know it's not welcome so I'll bow out.
Not interested in hijacking the thread, but glad to hear others have shared the experience. Mid 30s, just starting to realise the impact it's had, I felt the need to chip in...
To the OP (and anyone else in her situation) I'd say what I'd say to any parent: a relationship is a two-way street, yes. But as long as the person(s) involved are children, you do have to take your responsibility as an adult.
When do you ever see a parent ask for advise like "help, DC are doing my head in" and get a reply like "oh dear, sounds dreadful, but hey - those kids need to take some responsibility and be nice/respectful/considerate to your needs, after all you didn't sign up for parenting badly behaved children!"???
Because you are the adult in your relationship with the stepkids, your DP has their own relationship with them.
You are responsible for the quality in that relationship far, far more than the children. (Or your DP.)
If you find that difficult to deal with / comprehend, there's not a whole lot anyone can say or do to help.
And to anyone in the OPs DPs situation: REALLY? You choose to spend your life with someone who has such a low opinion of your DC? How do you think that'll make them feel? About themselves? About you?
Finally - LOVE. You can give it freely, and still have more than enough to go around.