I had that less than a month ago, when my first boyfriend broke up with me. Granted, this is very personal and everyone deals with it differently buy:
Crying it out and mourning helps. It's cathartic, like Snowguy said- if I didn't break out crying like I never did immediately after
the talk, I'm seriously not sure what I would've done with all that pain. Try to figure out which of your friends are truly, seriously supportive and talk to them- talking to friends truly helped me, and basically saved me a few times from doing something stupid like sending him a message. This is priceless. If there's no one irl you feel you can trust with your grief, there's no shame in turning to people in the internet, ie PMing people on Atlas, and I'm sure that many posters here (me included) would be glad to talk to listen to you.
What I found useful in getting up is griping an emotion other than sadness- for me, it was anger and determination: some "friend'' from univeristy posted a very hearty collage of pictures of him and my ex on his instagram story days after the breakup, which got me seething. Then there were the final exams, which I decided that I will succeed in and study for. If you have anything important take it up use it and work hard for it, and if the circumstances of the breakup give you reason for anger, don't be afraid to feel it (as long as you don't act on it, obviously).
Finally, at first you will only remember all the sweet memories with her, everything good that you will miss. It's understandable and hard, but try to think about the less-good things, the reasons for the breakup. In the end, whatever the case- whether you broke up with her or she with you- you deserve better, someone who will feel the same for you as you do for her. Try to tell yourself that.
As for a different girl, like a few posters suggested- personally, I found that impossible before I got over him. One night stands will just feel bad, and your heart is unlikely to grow attached to a new girl before the old one is no longer inside it.
Edit: I see that 10 days had passed since you posted that, but oh well, hopefully some of that will still be useful
