I feel for you dude. I was in the same situation, although the ex wasn't coming back and nothing serious was happening. All I can say is to keep your expectations low. You can't force a relationship with her. If you do you will go crazy and lose any chance of it happening. If it happens, it has to happen naturally at her pace. Keep some distance, make her chase you, and try meeting new people.

At the same time, you are 18. I'd honestly suggest dating around. I know how hard it is when you find someone you really connect to; nonetheless, you're not looking to get married, right? Socializing and dating are learned skills. You were shy before, but I suspect you have a ways to go in social confidence. Dating around doesn't mean getting in a relationship, but just meet some new women, get their #'s, and ask them to hang out.