Why can't i get close to people?

I just can't do it.. I mean emotionally close, (not physically). Whenever I think of having a boyfriend, I think of the end and that we're "gonna break up anyway" and I can't fully give myself to someone emotionally because if that happens then something bad will happen-Like I'll get my heart broken or he'll just randomly leave me, or he'll cheat on me or something... In psychology we watched a video and it said that guys (even if they live with a woman) will look for someone better... So what's the point if you are completley in love but a guy will just wander off? But no one else (that I know of) has this problem. What is this problem? Is it just experience from past relationships? Thanks for your help (I'm 18 year old girl)