Is it normal to want a girlfriend, but then... not?

OK my whole life I have never actually had a girlfriend for at least more than a day. Every time it gets to the point were it feels like we are a couple I don't want it at all. I'm not just trying to get laid. Its just that when it gets to that point i just don't want to be associated with anyone so I call it off. Then the cycle begins again. By the way I'm 21, not a virgin, or homosexual for a little more clarification. So is that normal?