Everyone wants what they don't have.
I made my own afc thread a year ago:
http://www.sosuave.net/forum/showthread.php?t=126708
Now I have a new gf who is ten times better. The old girl is single and would easily take me back, but now I don't want her back. She is a fine person and I wish her well, but I have something better.
I am telling you this because this story is very, very common. After you have a few relationships, you will see how girls come and go. Like a dog dying, it's sad, but you eventually just go get another dog. With time, you will gain some perspective and see that your oneitis is only psychological, wanting what you don't have, wanting to feel like a winner and not a failure. It's all in your head, and will pass with time...and new pvssy, that helps too.