Anzalone, I agree with other members that you are both far too young to get married. I realize this is hard advice to swallow. We were all your age once, and when you are young it's hard to listen to what you don't want to hear.
Regardless of your choice, I will give this one piece of advice. If you choose to forgive and stay with her, then it means you need to move forward. No throwing it her face when you are mad at her, accusing her of cheating when she is out or late, checking her cell phones.....etc.. Once you tell her you wish to stay with her and forgive her, then you no longer have the right to punish her.
If you feel that you cannot do this, then you need to move on. It means you cannot get past it, which is understandable, and a healthy relationship is not possible. Otherwise, you will continue to be angry at her, hurtful things will be said and done, and you could both wind up with some real emotional damage.
Do not let yourself get caught up in constant angst and chaos. When you are young and in the throes of a first powerful love, a bad relationship can become normal to you and a blueprint for future ones.
I do disagree with the "once a cheater....", especially when someone is young and makes unwise decisions. People are human and they mess up. We all blow it in some way at some point in our lives.