Relationship advice

I've been with this guy for almost 5 years now. He has given my an engagement ring but I have never been completely sure he is the one. While in the relationship I have put on a good bit of weight due to being depressed and he won't touch me. When we first got together we had sex very often at least one a week. Now I'm lucky if I get it once every 3 months. I practically beg him to touch me and then if we do it just doesn't feel like we are a part of each other anymore. I don't know what to do. I'm scared to throw the relationship away because of that. That physical contact is what makes me happy and what I crave. What should I do?