I can see how hard you’re trying to accept the situation but I too think you’d be better off walking away. For many reasons and none of them include you being selfish.
A few things jump out. The children don’t take anything away from you and they don’t turn up to deprive you of your freedom or spontaneous weekends. One of the shittest things people say to step parents is they knew what they were getting into and it’s not true at all. No one ever says that to a stressed or exhausted parent. But you did know he had children and that means a lot of the things you’re struggling with are inevitable. I don’t think it’s unfair he has them every/most weekends. We have DHs DC every weekend and during the week. He misses them, I miss them, they miss us, he’d rather be with them all the time so we have them as much as we can. That means we have less time “to ourselves” but it’s how we want it and we make the most of the time we do have wheh it’s just the two of us.
My husband never had any expectations of me other than to be kind to them. He’s their parent, he chose to have them, he is responsible for them. As it’s turned out I’m incredibly close to them and we’re our own family unit, I love them to bits and miss them so much when they’re not here. But all DH ever is is grateful for my effort, love and care. He would NEVER call me selfish and he knows how tiring they can be so he’s the one who says he’s taking them to park and leaving me to have some p&q for a couple of hours. I’m the bonus parent figure, the one they’d cope without, the one who gets to do the fun things. When I do the drudgery, because I choose to, the sick days off with them, the endless laundry, the meals, the cleaning, he’s thankful and appreciative. I want his life to be easier because I’m in it and he wants me to have the wonderful bits of family life.
We’re having a total bitch of a time having a baby together and it’s taken a toll on me exactly as you say with the hard, tiring stuff of parenting without the immediate joy and gratification of my own child. I always knew I wanted to be a mum and there are times when this halfway house is tricky. Other people struggling to conceive or with miscarriages can book a nice holiday, enjoy the freedom of not having responsibilities to help them through the struggles. We don’t have the time or money to do that, I’m still tied down by parenting week in week out!
What makes it easier is knowing what a strong unit we are with his DC and how much DH wants another baby as much as I do. If during the shit of the last few years I’d doubted it or felt that he was less bothered because he already has DC I don’t think we’d have made it.
You’ve got your whole life ahead of you. You’re not at all selfish to wish things were simpler and easier. You should be with someone who adores you, values and appreciates you, wants what you want. He’s going to want to see his DC as much as he can. You’ll feel the same when you have your own. But it’s not what you wanted from the start and he talked you into it and is now shitting on your efforts and your feelings. That’s not okay. You’ve given it a good go and if the two of you aren’t on the same page then there’s absolutely no shame in deciding you want something different.