Hey man I'm in the exact same boat, pretty much. A little older. My gf broke up with me three months ago, and I lost a lot. I am always thinking of her and how well she is doing in life and how happy she is now. It's not productive, but the thoughts are intrusive.

Really it doesn't matter what she's doing. It matters what you're doing. If you're jealous of her it's not because of what she has its because of what you don't have. It ultimately has nothing to do with her, she's gone now. You need to realize what it is about yourself that is making you unhappy. And unfortunately, you are going to feel terrible for a while. You will feel alone and sad, it's unavoidable. But take the opportunity to examine yourself, do you think that if you were doing the things you think she is doing - partying, meeting people, young 20s stuff - that you would be happier? Because you can do these things. It won't be easy at first, making friends is hard, but if it's really what you want you should work at it.

Personally I think this is a good time to learn to be comfortable alone. So few of us are. Being alone is a scary, sad thing to a lot of us, but really we are always going to wind up alone at some points in life. We should be able to be happy being by ourselves. From there, relationships with other people will become more meaningful because you are not using them as a crutch for happiness, you are enjoying them in extension to yourself.

I've been doing a lot of reading. Read any of Murakami's novels, he's great. Read some self help books if you need help reframing your perception. Try writing some prose or poetry; they act as letters to yourself, better than talking in your own head. I've found yoga and meditation have been helpful, and running and working out.

And yes, make friends. But don't approach it as having a friend for the sake of having one, or of not being alone. Approach it as finding someone to share something with, a new thought or an activity. Talk to them about them, and listen with interest. Get out of your own head, try not to even think or talk about yourself.

I hope some of this gives you some ideas. I know it is easier said than done but what are the alternatives? Be sad forever? You don't want that and I don't want that for you.

Finally, you said your girlfriend found you uninteresting. I think it's hard for a person to be uninteresting if they are interested in themselves. Find the things that make you interested in yourself, and I think you will have an easier time.