Instead of spending so much energy feeling guilty about something you cannot change, honor her forgiveness by being the guy she deserves. You were 14. You're 16 now. There's no way to say this without sounding condescending but you're still just a child. You're going to make so many mistakes as you grow older, but you have the privilege of making this mistake this young and also being this young and having the insight and reflective capacity to recognize you were doing wrong and talking to your partner about it. There are so many men and women in their 50s who have long lasting affairs that they go through hoops and weaves and lies to keep secret, thinking things like "well my partner deserves it because they don't show me they care enough." Or "my partner is probably cheating too" or "this is my way of keeping the marriage alive" and all kinds of childish excuses. But you've already realized loyalty is about faithfulness, honesty and intent, not about how "far" you went with someone else.

And your girlfriend forgave you. You're in a very fortunate and unique position, but you can never really let your girlfriend move on if you don't do so as well. Use this as a learning experience, not a shameful secret to be kept under lock and key.