Why can't I be in a long relationship?

Basically I've had a disgy past with men, some of my exes include prostitutes, cheaters, phsycopaths, manipulators and also 2 years ago I was raped. Anyway I would like to thing I've gotten over it all and I've found a really great boyfriend who I love, I really do love him, but I just can't put down my barriers with him...I just can't, I've loved before and I've ended up heartbroken, I've trusted before and I ended up raped. And because I'm so shut off from him it's causing me to push him away...I don't wanna loose him but I just don't know how to open up to him...;( what can I do