Question for girlfriends/fiancées of men with children

So I am a girlfriend of a guy who has a six year old. I have a 21 month old from another relationship. We’ve been together for almost a year and moved in together. We have this cute little blended family, I really enjoy it but I’m always questioning my boundaries. As I said, I’m only his girlfriend, not his wife so I am still worried about bonding with his daughter. There’s a specific way I like to discipline my own child, foods that I like to feed my child, activities that I like to introduce to my child. Etc. I want to involve her as well but she’s technically not my “step child” so i don’t want to overstep by doing things a different way. There’s so many ideas I have, that I’d do if it were my child. I’m not worried about not being allowed to offer new ideas, because my boyfriend is very receptive and appreciates them. But my issue is that I’m worried I’ll be taken an advantage of, if I take her under my wing. It sounds so dumb but I just have this fear of getting closer to her, and feeling like a step parent to her, but then it ends up not working out for some reason and I wasted all of that time and effort. It’s just so hard being the outsider and wanting to give your all to the child but then not knowing if it’s going to all be for nothing. The best way I can describe is I WANT to be closer to her but I’m scared to do it. My failed relationships in the past have made me very reluctant to trust someone to stay. Even though he makes it obvious he wants me to. The problem is more so with myself than it is with him or her and it doesn’t help I’m just a girlfriend. Does anyone else feel this way and what should I do?