I cheated on my gf with her ex.
What made you cheat on your gf with her ex?
Because me and and my gf are not working out.
If you're having troubles in your relationship, communication is the key to keeping it alive. Cheating only serves to hurt others and doesn't allow you to move on.
I do what I want and she should do the same if she knows what is good for her.
You still have a relationship with your gf and being with her ex is a breach of her trust and its hurtful to her. Breakup with her before you get into another sexual relationship as that would be the right thing to do.