I've been going out with her for over a year, and the relationship itself is great. It's just that I torture myself constantly and as a result, I've never been truly happy for long. I would worry about breaking up, or cheating or whatever. After one thing is over, my mind just invents another thing to be worried/unhappy about. It's almost like there's a little guy in my head that doesn't want me to be happy in a relationship!
You say that the relationship is great, are you sure about that?
Ps. Similar question to wolfgangs.