when you are hurt by your girlfriend due to she having an afair with a friend of hers for about 3 months, then you have proof and she still denies it until you have a chat with the other guy on the ...
The first thing I need you to understand is trust is a CHOICE! It is something you can choose to do. You can choose to give her the benefit of the doubt or you can choose not to. This is entirely in your court and not hers. She definitely needs to re-earn it, but if you want to give her another chance then you need to consider taking a chance. The only way your relationship can have a chance is if you put your hand out. If you can't do that, then it's time to move on.
If you can, then you have to be prepared for the shortfall but hope for the best.
It is a hard thing to trust after something like this, but you must try to see beyond it if you are ever going to give her another chance.
Something else you need to understand is this: If you are going to give her another chance, then you are, in essence, forgiving her what she did. This means that you must not bring it up again unless the situation re-arises. You may not be actively forgiving her but you are giving the appearance of forgiveness. To bring up her failings from the past is like using blackmail on your loved one. It's a rotten thing to do.
I hope you can work it out but I also hope you use common sense and listen to your heart and instincts. You know her better than any of us here on Yedda. Choose wisely and live well!