Unless you have proof that she is cheating, getting jealous and over posseive will just push her away from you. I would suggest that you sit down and talk to her about your problem.
First thing ,Get a couple of your friends . And like you know form a type of groupSecond Thing ,Get a new GfThird Thing ,Ask your ex to join your group( If she doesn't she is over you , if she does...
If you are afraid of being ridiculed by other people, that can definitely be hard, and I think everyone goes through that where they fear being called names or being mocked because you don't have a...