Letting go of the past?

Long story short... I don't know what it feels like to be in love or to feel happiness in any form. It's like all my senses have been numbed, but I don't have a problem expressing negativity and anger. My relationships don't last because after a few months i lose all interest in sex. I am in a relationship now and my partner has gotten to the point where she can't handle us not having sex anymore. She has become a miserable person and it's all because of me. We get on really well together and actually love each others company, but when it comes to being intimate, I just can't or won't allow myself to just be happy. I need help otherwise I'm gonna lose the person I know that is actually good for me... I was in an abusive relationship years ago and ever sense then i have had this problem... I think I am protecting myself to my own detriment