Why do I suddenly get cold feet?

So I like a boy I pursue him and finally get him I'll be head over heels in love and then BAM. I feel repulsed my boyfriends are always so nice but I have a problem they are fallin more in love with me and i'm pulling away. What's my problem I don't even control ot please it's ruined past relationships and I want to save this one. why do I do this?