A few things could be the reason for this.
One is scarcity. Or at least the perception of scarcity. You say this has happened the last 4 times, I learned after my first. Although I still get feelings, but not as much anymore. Sometimes none after a while. The idea here is abundance. You want to get it. Why? Because with abundance come the traits of what you actually want to be. A cool dude who has fun but doesn't need it because he knows he can get it anytime he wants. And when he does want to get it he gets it with pure intent, unapologetically, and is unaffected by whatever happens with any one girl. With abundance you know you can get new pussy, and therefore this one here is great, but you don't NEED it.
The second is emotion. After a few heartaches you should be teaching yourself not to touch the burning hob. At least not for a while. Let it warm up, first. Have sex, and keep it purely as such. Then see where it goes. You don't want to need her more than she needs you. This gets her chasing you. Now, how do you get there? It's a little bit like distancing yourself emotionally. Not completely, because emotion is a huge part of pickup. However, distancing yourself emotionally from her validation. You want to completely get rid of your validation seeking attitude. Sometimes you learn it by reading on these forums and applying the mindsets, but a lot of the times you learn it through real hard experience. You get hurt, you don't want it to happen again. But you want pussy. What do you do? You go for pussy without INVESTING too emotionally in the girl. Invest less in her than she invests in you, or at least be MUTUAL about it.
In conclusion, go out more. This will get your mind off her. You probably know what you must do. In case you don't:
http://www.youtube.com/watch?v=YNovswAlmio