Okay, let me give you some advice: if a girl pulls the breakup card on you, do NOT make yourself available to her over the next week. Do not try to convince her to stay with you, do not plead with her to spend time with you to "talk things over", do not discuss friendship after the breakup. Going to the movies with her is a huge fucking mistake on your part, and you really ought to pull out of that if you can.

Stay away from her for at least a week. Ignore her calls and generally blow her off. Change the locks if you live together. (Let her in if she needs to get anything from your place, but don't say a word to her.) If she's serious about the breakup, this won't do any additional harm (she already feels bad and that she "deserves" this). If she's not, then this is what's known as a "shit test"-- she's unconsciously testing you for how you respond to bad behavior-- in which case you must let her know that the break-up-and-get-back-together cycle is, under absolutely no circumstances, acceptable, and that she risks losing you forever if she tries it again. Become so distant to her that she never does this to you again. If you make yourself available to her after she pulls the breakup card, you're basically rewarding her for being bitchy and she will misbehave again.

It will be a miserable, hellish week for you, but consider this to be inoculation against a permanent breakup, which is inevitable if you let her get away with this. (Bottom line: if you let a girl walk all over you, you lose her respect, often permanently.) Exercise. Get outside, even if it's -10 degrees, just to get some sunlight in your eyes. Work, or at least try to do so, and if you can't, take a walk. Meditate. Go out with your friends and meet new people. Consider this an opportunity to train your mind and soul in happiness despite adversity. Enjoy it as much as you can, though it's not easy. This is one of the most difficult things we experience, this side of sickness and death, and it's pleasant for no one.

Regarding post-breakup misery, my experience is that it peaks very early. After 3 days, the worst is pretty much over. After about 2 weeks, you'll be back to your normal happiness level, except when you think about the ex. It'll probably be 2-4 months before you can start dating in earnest again; rebound relationships considered harmful. You shouldn't try to be friends with the ex until 6 months have passed. Also, know at all times that time is on your side. Breakups hurt the worst at onset, so you will feel better every day. By the end of your week apart, the worst of your post-breakup misery should be over, and you should be able to function normally. Then, if she comes back to you wanting to resume the relationship, then you will hold the upper hand, whereas if you seek her out or let her choose when to have "the talk" about getting back together, she will. Blow her off the first time she wants to have "the talk" and make her wait at least a week. You're busy, and you were spending time on other things. (Sorry to make this sound competitive, but the fact is that she is either serious about the breakup or she is not. In the former case, it doesn't matter; in the latter case, she attacked you, and you need either to make the decision to break up with her, or to win the fight she started.)

One thing I remember about my most recent and hardest breakup is that the fond memories came flooding back and were a source of intense pain, because I realized that I was permanently separated from them. An insight I learned from my Buddhist meditation practice was that those memories were no more "gone" with the breakup than without. Or, put another way, they'd be just as much "gone" had the breakup never happened. They exist only in the past. The best I could get by continuing the relationship would be the potential to have more beautiful moments... but, of course, I still have that exact same potential, albeit with someone else. This made me realize that a breakup is, on the grander scale, not very significant.