People generally don't leave like that on a whim. From what you wrote, it appears that you thought the relationship was fine. She obviously didn't. That means you, as a couple, haven't been communicating well. Do you know why? Try to figure that out. As Kel said, try to listen to her without arguing or reasoning. Ask her to please tell you how she feels, and that you'll just listen. After that, ask her what she would need from you for things to change. Just take it all in.

Realize that it's very likely that Amy has tried to talk about this with you before, but that you weren't open to it (maybe because you didn't realize what she was trying to say or because she's not very good at conveying her thoughts and feelings, I don't know).

It's understandable that you are so hurt that you are turning defensive to the extent that the next day already you sort of feel like moving on and giving up on the relationship. That's the hurt talking. Don't let hurt guide you, it's a poor guide that never leads to happiness.