Do you take any medicaments like propecia or something like that?
I heard these can cause stuff like this.

If not, it's most likely some psychologic issue.
It could very well be that you project your failed relationship on other girls and possible future relationships. Perhaps even subconsciously.
And as you said you can't really have sex without feeling something for her that might be a problem.
Take some time and think about it, try to feel, and name your feelings. Maybe you can spot something.
But then again, I'm no expert. It's just a guess. KITTYCAT.