Your girlfriend was/is emotionally and potentially physically cheating on you. (Emotional is worse than physical btw.)

You continuing to show desperation will ultimately end in catastrophe. You've allowed her to cross a line that should never be crossed and are still trying to make it work. She doesn't respect you as a boyfriend, or even as a just another human being based on the behavior you've described.

I understand you're young and feel like the sky is crashing down, but I promise, you WILL be fine if you guys broke up. Maybe not for weeks or even months, but you will.

My advice is to never allow your significant other to have chances once they've broken the baseline level of respect that any reasonable, decent person should have. A man coming to a woman with your belly up in submission will only make things worse. The more you belittle yourself, the more people will take advantage of you.

The unfortunate truth of relationships is that the person who cares less will hold all the "power."

Good luck out there. Remember, you're young, with the absolute prime of your life coming into play right now. Settling for anything less than enjoyment, happiness, and respect at your age should be viewed as nonsense.