Most Helpful Guy
The pain you're feeling is normal, and you should embrace it as it is a sign of how much your relationship meant to you. The fact that he kept his feelings for this other girl from you for so long is probably because he had no idea how to tell you without hurting your feelings.
As for what this other girl has that you don't, there's probably no real answer to that. The qualities that you love about one person can be qualities you absolutely hate seeing in another. In the end it's not about how attractive you are, it's who you are. If a relationship doesn't work it's simply because the people involved aren't a good match, or not yet (but don't get your hopes up).
To deal with it, allow yourself to feel sad and cry when you need to. Talk to your friends and keep yourself busy with things so you don't constantly feel like you need to take some form of action. Physical exercise helps too, especially running or even taking a walk. Finally, allow yourself to feel things for other people. If you get asked out, consider saying yes (if you feel some sort of attraction) and try to have a good time.