Not sure I want this life any more

Me (F, 27) and my partner (M, 31) have been together 2 years, our relationship has been a whirlwind, we fell in love quickly and we are recently engaged. I am head over heels for him , he has a child (9) and I have always struggled with this. His son stays over every weekend and occasionally a day in the week. I do want a family of my own , but I do struggle with being a step parent and I don’t feel like I have a very good bond with my step son, I feel like this is partly my fault as it’s not something I truly want (without sounding awful, all children are incredible and it’s nothing against them). I love my partner so much and have never had a relationship like this , but I do feel it’s holding me back. Does anyone have any advice in this situation Thanks