You can be curious about why, but in my experience, that’s a losing proposition. There never really is a good explanation. You’re just dragging it on by trying to ask her.
She actually did you a great favor by cutting it off the way she did. Believe me. Most women will drag it out and try to dump you nicely. There is no nice way. It’s better in the long run to do it fast and hard and with no explanation. You will get over it faster if she keeps refusing to have anything to do with you. Truly, she is brilliant! I wish she had dumped me instead of my college girlfriend. Oh man. There’s a horror story.
You must now grieve this relationship, and that will take time. How long? No one can tell you. It takes as long as it takes for you. But knowing you are grieving a loss can help. You will think about it all the time at first. Slowly, as the months pass, you will have more and more time free of thoughts of it.
Try to keep busy. Try to hang out with friends as much as possible. Being alone is when it is the hardest. Do not try to drink it away. That’s no good. It is best to feel it and feel it and then be done with it. If you want help, see a therapist. No shame in that. You should focus on rebuilding your new life. There is nothing to be gained by trying to fix the past, and little to be gained by trying to figure out what went wrong. You have to give up on that, I’m afraid. You can’t figure that out without her, and she isn’t going to participate if she’s wise.
I’m sorry. It’s tough. It’s tough love, in fact. But really, that’s the best way in the long run. It took me two years to get over a year and a half relationship. She kept visiting me for a kind of fwb thing for a while. That was so painful. I couldn’t say no, but she kept rebuilding my hopes. I guarantee you that a sudden, complete cutoff is far better.
Hang in there. It hurts like hell and will take a long time, but it will end.