I can't say I'm truly happy right now because DH and I are having some issues (not SC related) but in terms of the stepparent relationship I am happy. I never used to be, I really struggled in the early days but things that have helped me are:
Leaving all communication with the ex up to DH. I used to see her when she dropped DSD off. There was clearly tension there and it was awful. I don't see or speak to her any more and that distance really helps. Also not getting involved at all in what's been said between them. If DH wants to discuss it I'm happy to listen but I'm not happy to get dragged into drama or tbh even really offer my opinion unless it directly affects me.
Space - allowing DH and DSD time alone. This is a two fold thing - it gives me time to myself and also allows them to strengthen their bond. In the early days I felt really insecure and felt I had to be around all the time. This negatively affected me (we all need space sometimes) but also negatively affected their bond as they weren't getting enough alone time.
Focusing on the positives. As a childless stepparent, I found this hard. Kids can be annoying and push your boundaries. But they also have lots of positives. Reaffirming the positive in my mind and out loud definitely helps.
Choosing a hill to die on. As an example, I've given up caring if she dumps her clothes in the bottom of her wardrobe instead of folding them. If DH doesn't care then that's up to him. Small things like that are not worth stressing over and I'm not prepared to argue over it when in the grand scheme of things it's trivial and just a case of different approaches rather than anything detrimental.
Recognising DSD is her own person and that the negatives don't always come from Mum. DSD has her own views, thoughts, ways of dealing with things etc and just because they may not be the way I'd deal with them or the way I would think, it doesn't mean it's wrong or that it's solely come from her Mum or that any DC I give birth to would be any different.
Spending time with DSD on my own, making time for her and time to build our personal relationship. I think this was key in focusing more on the positives as it gives me a chance to talk to her, get to know her and just generally build a positive relationship.
Honestly though I think a lot of it depends on the personalities of everyone involved. If you have a high conflict ex, a disney Dad, difficult communication in your relationship etc then all these things make the step parenting role a lot more difficult. It's not easy but it doesn't have to be made harder by those around us.