I've never been bothered about having my own kids and sometimes do resent the assumption that I will automatically step up and be a parent at weekends....I haven't got a clue.
Ditto! As a stepmum to my DP's 6 year old, and someone who has no kids of my own and not really a "children" person (or at least I wasn't), I completely sympathise.
Am [hshock] that you get up at 7am and do the nappies, let alone without being thanked! I must admit I am very rarely the one to get up with DSS on Saturdays and Sundays (he's always up and about and demanding breakfast and company at around 8am) I generally leave it to DP and stay in bed myself. Am happy to be the one to get up early ocasionally in order to give DP a break, or if there is a good reason he can't do it that day, but regular 7ams + nappies + no thanks...I'd be very resentful!
I'm lucky in so far as DP and I are both on very friendly terms with his ex and her DP. But I think maybe whether you are on good terms or not with the ex, it can still be hard to accept that they will always be in your life and have an influence on it. Its very hard for me to accept that DP, and therefore I, will always have to live close to wherever ex chooses to live. I sometimes have fantasies of us all sitting down together and me saying "well I'd really like to live in town X, can we try to negotiate this?" but even given our friendly dynamic I think this is a dream too far ;-)
Anyway, I don't really have any wise words - I totally understand the thoughts of "it would be easier not to be with a man with a child" but at the same time, if you're anything like me, your love for your DP probably outweighs the negative stuff thus you don't want to leave.
My only real pieces of advice, which have worked for me in countering the resentment issues:
a) keep on playing with your DSD and trying to enjoy her as much as you can. Sometimes at my most resentful I avoid more than a bare minimum of playing/talking to/looking after DSS, thinking it should be his dad doing it all I and I should be able to get on with whatever I want to be doing. But ultimately this always ends up leaving me feeling worse. I'm happier when I actually throw myself into the SM role a little bit more (within limits ;-) ).
b) Try to detatch as much as possible from issues of disciplining or making parental-type decisions, particularly insofar as when your idea of appropriate parenting differs from DP's and/or from his ex's. I try to remember no child will ever be perfectly behaved, so even though I sometimes think I have the answers as to what should be being done differently, it probably wouldn't make that much difference and its easier just to let the bio parents get on with their own parenting. Just take deep breaths and try to distract your mind in situations where your inner mind is screaming "but this is stupid, I'd never make this decision!" and only intervene if really necessary.
To be honest you sound like you are already doing a great job. I just hope you're not doing too many 7ams ;-)