Hey man, things are rough all over, sorry you're having a hard time of it. Here's some perspective.
You remember before you met this girl? You knew, KNEW what you were about and where you were headed. I'm sure you had a pretty good idea of what things would be like for you and couldn't imagine meeting a girl that would change your life. Well, now you know you were wrong. That's how life is, it changes in front of you, you know where you going and then all of a sudden you're somewhere else.
This is good.
All anyone can do during their time in this infant universe is live life as it happens. That's what you're doing right now. You'll never get over the experience you've had with this girl, that's not how people work, you'll always have that experience. That's part of who you are now and you're better for it, you think you hate yourself but what you're doing is experiencing your regret. Regret is a powerful thing, it comes from not taking the actions you decide in hindsight you should have taken. This is an important thing to pay attention to, the only way you can feel regret is through learning, you've learned that who you were with this girl is not who you wanted to be. Congratulations! That is growth in it's most fundamental form. You are not today who you were a month ago and that regret? Those scenarios you run through in your mind? They could never have happened. Not in an alternate timeline, not in a parallel dimension, because the only way you can look back is by living through it, the way you get to today, feeling how you feel is by experiencing what happened to you yesterday and the day before and the week before that and every moment you're eyes have been open since the day you were born. We are the sum of our experiences, what you are going through now is hard, I know and it won't stop being hard but difficulty and pain like this is the only way we can learn who we are.
When someone says "it is better to have loved and lost than never to have loved at all" and they're not reading from a book, what they're telling you is that it's a better thing to live a life that knows the joy of caring about something more than yourself and experiencing the ways that changes you as a person, than to have never felt those emotions, made those mistakes or learned those lessons.
The next time you feel like you can't get out of bed, lay there, eyes closed and try to wiggle your toes. When they move you know, instantly and irrefutably that you're in control of your life. Your life. That sum of those experiences that you wished were different, all of that is behind you and has equipped you to never make those mistakes, she's not dead. You're not dead. You're both young, when you can, thank her for your time together, you don't need to move on, or forget the time you had, you just need to accept it into who you are and get ready for the next thing to come along that you "know" will never happen.
It's alright, everything's gonna be okay.