Do whatever is necessary to take care of your health. You are brave to seek help. Don't discount that. About your relationship, it's very normal to feel as you do. I married a man who had a 4 year old. Due to my family of origin I felt very afraid and incompetent about raising children. (I didn't have any of my own) However, I think you'll discover that because you love that child, you'll be an amazing parent. Just being there for them is the BEST! Listening, cradling, loving, and being available is so important. You'll know when and if you're ready for marriage. Don't push yourself into something. I would imagine that in therapy you'll discover things about yourself that will help you to make those decisions. I wish you ALL the BEST in everything! :-)
Try thinking, women, not girls just women.
Okay, about the kid. I'm assuming ever parent feel the same. Like they should do more for kids, or they are not adequate. No parent is fully confidient. And the wedding, maybe you don't feel comfortable with it ? Even though you love him. Maybe a wedding is not thing. Regardless in order to be a good parent and a wife you must take yourself to be there for them. So make sure you are okay and then you can make sure you can take care everything else.