Why can't I get emotionally 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. Whenever I do say something personal I feel very uncomfortable, like I'm sharing too much and I'll regret saying too much and opening up to the person. What is this problem? Thanks for your help (I'm 18 year old girl)