Okay, this is pretty common so, point one you aren't alone.
That's important to remember because forgetting that will make you feel like ****
Point two, you need to keep yourself occupied
I'm talking binge watching movies, reading books, study, cook/bake, art, gym whatever you want
Just do stuff, discover a new talent, try to learn something, do something that scares you
It's strange but it'll help a little
Note that you don't always have to go out, if you're happy to do things inside you should - sometimes you can be the loneliest in the biggest of crowds
Point three, if you can, replace the feeling
It's a bad coping technique yes but it helped a lot of my friends and me, just pick a person (celebrities are safest because what are the chances of you actually meeting them) and just pine over them for a while
Sounds stupid but there's entire articles over how 'love' of somethings almost acts like a drug and having something change in that almost makes you acts as if in drug withdrawal so it's time to feed the addiction
Point four, internet buddies!
Anonymous internet buddies are best types of buddies
Like this, you've asked for help and you shall receive
Or at else you'll receive some sort of reply
Try 'blahtherapy' if this doesn't work out for you, they have a whole host of relationship issue forums over there
Point five, remember you had a good time but you need to let it go
Hate to be that person, but nothing lasts forever
It's not the end of your dating life as we know it, chances are you'll fall just as hard for another girl in the future
So learn from your mistakes, yeah?