Some advice regarding how to deal with a breakup.
First of all, it is ok to feel grief and to feel down, and crying is often a part of the process. Accept that you feel like you do, and realize that it is ok. You will feel like shit for a few weeks, but things will get better. Try to eat, even if you probably don’t want to. Things will only feel worse if you are hungry as well. (It is kind of ironic that love both starts and ends with a lack of appetite.)
After a few weeks
After being a mess for a few weeks (1-3 weeks or so), you will have to start the process of moving on.
Everything that reminds you about her (photos, emails, that thing she gave you, stuff that is actually hers, etc., etc.), should be packed and placed where you can’t see it. You could throw it away, or you can just put it in a box and place it somewhere you rarely are. It should not be placed under the bed or in your closet where you keep your clothes etc.
Next thing is to change stuff. Buy some new clothes and shoes, change stuff in your apartment/house, change your haircut, etc. etc. You know best what is ok to change and not. It might be as small stuff like sleeping the other way in your bed. In fact, this is a good thing, since it is probably hard for you not to think about her when you go to bed. A new perfume might also be a good idea, since you likely will associate a specific smell with things in your past.
There is probably stuff that you have thought about doing before, but you never got to it. This is the time to do them. Right now. Go to that painting class, learn to dance, visit that old friend, run that marathon, or whatever you might have been thinking of doing. Start with one or two things, and then add things if there is room in your schedule.
Get support from friends
Invite friends. Hang out with friends more than before (I know that you didn’t hang out as much with friends as you did before your GF). Your friends are well needed in times like this. And they are perhaps the only ones that will tolerate your rants about your ex.
Get out there
Go to parties, say yes to invitations for all kinds of activities, this is the time to try new stuff. Meet some new people (but remember that your old friends are the ones that you should prioritize, they are the ones that will actually be there for you). New friends are good since they do not know you and your ex as a couple. Thus, they will not remind you about her, and the will treat you as if she never existed (because she never did, for them). It is good if at least some of the new people you meet and hang out with are females. It is easier to stop thinking about your ex if you are hanging out with some really nice looking girls, even if they are just friends and nothing more.
It takes time
Prepare yourself for this to take several months. Perhaps 6 months, or even a year. It all depends on your relationship, and you as a person. But it will take some time. However, the pain you feel right now will fade away much faster than so. The first few weeks are the worst.
Letting go or trying to get her back?
If you want to get her back or if you want to get over her, the advice is the same. So you have nothing to loose here. Which means that you can start without having to let her go 100 %. And I know that you can’t let her go just like that. It just isn’t that simple.
A few things not to do
Do not talk bad about your ex in front of other people.
Do not call/text/email your ex too often. It is not good for either of you.
Do not become an asshole. I’ve seen many men treating all females like shit after a breakup. The random girl can not help that you and your ex are not together any more.
Do not tell your ex about any dates, one night stands, or anything else like that. It will only make her think that you are an ass. She will probably find out anyway, but finding out another way will maybe make her jealous, but she will not think that you are an ass. (She might just want you back, though. And if she does, you will have to know what you want.)
Do not check her Facebook profile. You got to trust me on this one, just don’t do it.
Do not beg her to take you back. Do not cry in front of her. Cry with close friends instead.
Do not call/text her while drunk.