Should I be comfortable as stepmom?

I’ve been with my partner 4 years a couple of days before we got together he had a one night stand which resulted in a pregnancy I’ve always tried to push him to communicate with his now nearly 4 year old daughter and they get in so we’ll I’m really happy for him however the girls mother has made our life hell for the most part of it fast forward to now we’re all in a good place and he’s got regular visitation but I find myself stepping back at visits etc just standing back letting them get on with their visits the daughter has got special needs so o don’t wanna overwhelm her however I myself am struggling with infertility I love my partner and his little girl endlessly but watching them together I sometimes get a little pain in my heart knowing I can never give him this kind of happiness I guess my question is does this make me a bad person

No, you’re not a bad person. I think because of your situation you should start slowly including yourself. Have a conversation with your husband, tell him you’d like to be more involved. They are your family