So recently me and my friend started going out (its long distance) problem is i actually said no even though we feel the same way i dislike the added expectations of being in a relationship so i know she's just going to be disappointed but she wont believe me even though I have told her how my past relationships all ended. I don't know what i should do just go with it or break it off and lose the only person keeping me sane...

This seems to be a tough situation for you, based on what you wrote your experiences with relationships didn't go too well. But there is one thing you should actually keep in mind and that is even though your previous relationships have failed, doesn't mean that this one will. I personally think that you should try it if you think she's the one for you, but you should ofc always follow your own heart in this.