It sure is good to offload, and to know it's not just you!
I'm really really lucky in that my DH and I are on the same page. Having said that, we weren't always and it was a little rocky to start with, as we had quite different parenting styles, and for a long time he bent over backwards not to upset his ex. Thankfully that stopped after a while, but it took years. She'd done a real number on him and his self esteem was in tatters. He'd got very good at putting his head in the sand and hoping the ikky stuff would go away. With the kids he had to learn to step up, and I had to learn to let go/leave it to him more (aka disengage). One thing tho is that we have always presented a united front, even if we disagreed (ok had free and frank discussions!) in private.
When we first got together he told me his kids were feral. I thought he was joking. Nope. His eldest (who was 8 at the time) was often in her own little world and everyone danced to her tune. Once when we were all seated at the table for dinner she came in, knelt on the chair and proceeded to eat the food off her plate with her mouth. I asked her what on earth she was doing and she looked at me as if I was stupid and said "I'm a cat". I replied that if she wanted to be a cat she could put her food on the floor next to the cat's bowl and eat there. Girls ate at the table with proper manners.
Another instance, around the same time, again at dinner, I had an big roasting dish full of oven chips (there were 7 of us). She put most of them on her plate. I told her that was greedy and to put most of them back so others could have some too. She threw THE biggest tantrum, crying, screaming... it was unbelievable. I said if she wanted to act like that she could go outside and stay there until she could behave. It took about an hour, and the others being told to ignore her (including DH who wanted to go and give her a cuddle - effectively pandering to the her) before she came in and apologised. For a while, once the initial dust had settled, I thought she and I might actually have a chance at a reasonable relationship, but that didn't last and I'm back to being the font of all evil. Water off a duck's back now, in fact, I seldom even think about her these days.
So yeah, DH and I weren't always on the same page, but we learnt. Most importantly we both worked at it and as a result we are closer now than we were when we first got together, and I think that's pretty rare. It also means that when we get blindsided by stuff (and boy, it comes out of no-where sometimes) we are able to handle it better.
Spangly you are absolutely doing the right thing. Your DP has to step up and grow some scoats with this ex, and parent his daughter. Hang in there.
Bananas your situation sounds hideous and you should be really proud of being able to be the bigger person and rise above it. You're right, you do need an emotionally mature DP.
Unfortunately some DP's are terrified that if they put boundaries in place/stand up to their ex, they will somehow loose their kids, and it's not until the kids are in their teens that they realise the damage that's been done, and by then it's too late. They've made a rod for their own back, and everyone else's too, and often along the way they've lost the one person who could have made their lives easier, their new partner.
Blended families are damn hard work and I think that everyone who takes it on board deserves a medal.