Dating Help?

So I like people and occasionally they like me back, ask me out etc etc... I have really strogg feelings for the person but as soon as Im in a relationship, my feelings for them fade within like 2 days. While they continue to be ridiculously sweet and tell me how much they like me, Im just over here ignoring them and eventually breaking up with them. Quite a few people think Im a jerk for that and Ive got a heartbreakers reputation. I just cant... like people when Im in a relationship with them. Help?