Hi Jtnz,
I don't think there's anything wrong with you, so for a start, dont' worry.
The difference between who you feel you can be when behind a computer screen is often very different to how you feel 'in the real world'.
So let's start with that.
From my own experience, I remember sitting inside spending actually too much time online, staying up stupidly late on forums, etc. I found that the more I did that, the more unsociable I became in real life. So although it may seem hard, I really would recommend just taking some time away from the computer, leave off forums and Facebook etc for a few days, and try to re-connect with the real people in your life. It may take a while for you to regain some of your confidence, but I doubt you've lost it altogether! You just need to shake things up a bit.
Are there any societies or group events you could join where you could meet new people?
Or any friends that you haven't spoken to in a while? Give them a ring!
Also, I know it may sound like the last thing that you wanna do, but doing some exercise and getting some fresh air will honestly make you feel better too. And hey, you might even meet some new people through it too!
Ok, now about your girlfriend....
Whatever she has done in the past that's messed you up - Have you thought about talking about it with her again? Does it still really bother you? Or is it just something that always comes to the front of your mind when you're feeling down anyway.
I think you need to think about how much it bothers you - So much that you think what who had has been tainted by it so much that maybe it's time to call it a day? Or is it something that you feel you guys can move on from?
Anyways, chin up, try to think about what you've got going on and the great aspects of your personality, and good luck