Most Helpful Girl
Sounds like both of you still have some growing up to do. You both seem to act and make decisions at the moment they come up, without really spending time to consider the situation or the other person.
On your part, you definitely made a huge mistake (but it's obvious you already know that). I feel like this decision was rash and not well-thought-out. Also, you left no room for communication with her; you just walked out. Had you talked this through with her, you guys could have probably seen where the weaknesses in your relationship were and at least addressed or started to work through those things, but since you just up and left without any explanation, you can't really expect her to not feel hurt and angry at you. I don't blame her for not wanting/taking you back after what you did, BUT this is also where the "fault" on her part comes in (of course, it's not REALLY her fault because it's ALL on you, man)...
On her part, her "mistake" (for lack of a better word) of saying that she's over you after only one day is a quick statement to make. This shows that she didn't love you enough to go to you and try to figure out why you did such a hurtful thing. However, I honestly don't think she was truly over you after one day. I do, however, think that she was extremely hurt by what you did, so to sort of get back at you for hurting her, she also said something equally as hurtful to you, which was that she was already over you.
My question to you is, have you tried to contact her? In what way (s) have you tried to contact her? If you did, what did you say to her?