Why do I feel this way?

The reason you feel like this is probably because you want what you can't have. Remember that you broke up with him because you didn't like him any more. And if he still liked you the way he did you wouldn't care, it's just the fact that he has moved on and you can't have him any more. Stop thinking about him. Delete any messages, texts, emails, anything off him. And don't talk to him. Leave him as a friend on facebook but hide his posts on your homepage..and his girlfriend's. You will find someone else, but until then just have fun. Enjoy being single and something will come along when you least expect it!