I wonder how many step mothers are responding to OP post?
I don't see that this makes much of a difference, but I am a step parent to 3 kids. Its hard sometimes yes, but I have never resented the children nor the attention they recieve, and honestly..I have always got on with them (bar some disagreements when they got older about random things but thats normal of course) and been happy to have them around. Though I admit it was also nice to get a 'break' bfore I had my own. Was lovely to see them, but also lovely to be able to chill and recharge for the next time!
I always knew I wanted kids though. Everyone thought I was crazy, for marrying a man who had 3 children already. I got on with them, I even get on with the ex..and its all a fairly decent situation and has been since the start tbh. The stepkids prepared me for having my own children too in a way, as obviously its different..but also sort of the same. Its only different in the way that..the stepkids were there half the week, where obviously mine are here constantly though really. Though now we do have his eldest daughter here permanently too due to a huge falling out between her and her mother. I try to treat them all the same, though do admit that sometimes mine get more (attention and financially) from me. But again, they live with me permanently where stepkids were split between us and their mother. I also figure that they get attention and stuff from their mother, which mine don't get as..well I am their mother. So I don't feel too guilty about it even though some people tell me I should and that its horrible to buy my kids something new if I don't also get for the stepkids, or if we are going out on a day where stepkids are with their mum then I should make sure I take them too (or even once, that I should never take mine out anywhere unless the stepkids were there!!)..etc etc. It seems stepparents are in the wrong whatever they do, and you get a thick skin from all the gossip really.
Mind, since having DSD here all the time, shes been 'even' attention and cash spent wise and stuff really, though I don't exactly keep lists to check. Wasn't something I have ever questioned. I know it sounds weird to some but I really do see them as my own in a way, even though I know they aren't.
Really weird relationships I think, stepkid/stepparent. Also theres always many people there to criticize or tell you you are doing not enough/too much. Everyone who matters (eg. me, DH, ex and the kids) seems happy with the situation with us though so..I don't care too much what people think/say. Both DSS know that they have a fulltime home with us too should they ever need it. And DHs ex is happy with it all also. Many thought she was awful for basically switching with DSD. Saying that it was fine for her to move in with us and she just saw her odd days or weekends in school holidays. But it works.
This situation sounds like it would never work, with OP. I do think, given what she has said in her post that she nows this deep down though. A relationship with a man who is a father won't ever work if you don't want him paying attention to his kids tbh.