Sorry, Scottishmummy I generally think your posts are very valid, but in this case, and probably because I'm in a similar situation but only with the 'other child' scenario, I'm with the OP. She's being taken for granted, and her dp is flapping his chicken head off over all the wrong things. Frankly, he doesn't know how to juggle his relationships, and why should he know how? I don't think I could do it either. I do definitely feel something for the DSD, she's 5 years old for christ's sake, she's reacting to her situation and it's a bit heart breaking, but OP took her on as a shared responsibility between herself AND dp, and his ex, but is taking the lion's share from both her dp and his ex, and she's not even hers!
If I sat down and really thought about it, however, I think I'd decide that the person who is still standing by me and supporting me should take priority. I do not mean to the detriment of the DSD, but definitely to the detriment of the ex.
Imagine, if OP removed herself and her daughter from him. He will have one daughter and an ex, and he will pay maintenance to a daughter he cannot look after, plus maintenance to a daughter who doesn't live at home. Two exes as well, so, what, a third woman starts up?
Soz for bad grammar, few beers and that.
Listen, please, OP, you are being taken massively for granted. Please keep the high ground, treat DSD as perfectly as you treat your own daughter, none of this is her fault. Try to see how you'd react if your own daughter was displaying this behaviour. You WOULD try to understand and figure out a solution. You resent the girl because she represents the shitty part of your life.
However, do NOT be a doormat any more. You've tried so damn hard to accommodate your dp's relationship with his daughter and his ex, and it is not your place. IMO you shouldn't take any responsibility over his DSD at least until he proves he could cope all by himself. If he can do that, then he's earnt his father's badge.
Keep the bond with your daughter strong, because if all else fails, you will still have each other.