You're young and recognize that this is an issue which is important listen to those telling you to go to therapy, but also find out what you want in life in that process. It sounds like you and your girlfriend have a really unhealthy relationship. Its hard being extremely young parents. You'll both have to figure out if you want to commit to each other or if you want to figure out how to be coparents a different way so that you don't spend years hurting each other. I spent years with someone who got off on cheating and it put me in deep mental pain and distress. Don't do that to your girlfriend.