Samnaomi,
Something that took me a really long time to realize may help you out. You certainly seem to have a lot of questions. What should I do, what if he does this, what if he does that, how do I react, how will I forgive him if he comes back, what if he's messing around with other girls, etc, etc, etc. I know what its like to have all these questions running through your head.
In three months, I have finally learned something which helps me to just not care, and calm down. You MUST realize that whatever he is doing, saying, is going to do and say is completely out of your control. Nothing you do will affect how he is going to act. Once I realized this, it is much easier to say to yourself: "Why am I going to worry about what I can't control?". Whatever he is doing, is not your problem anymore. Let it go, work on yourself.
I know that this probably won't sink in to you, but I'm hoping I can spare you some of the time it took me to finally discover this logic.