Break ups are most definitely very rough. You have to allow yourself to grieve the relationship, it's a loss, just like a death, and you have to allow yourself to go through the process. It takes time for sure.
The best thing you could do is keep yourself distracted, surround yourself with loving and supportive people, friends and family. Pick up a new hobby, meet some new people, just to better your life and your circumstances. Definitely don't rush into another relationship anytime soon, you need some time to get over this one first.
Obviously, if you're REALLY having a rough time coping, or feel you are at risk for harming yourself, it goes without saying that you need to seek help immediately. That's most certainly NOT the answer. As horrible and heart broken as you feel right now, you won't feel like that forever. Your life will go on.
We're here for you, keep posting and let us know how you're doing.
It's hard to be dumped. My boyfriend did so after 3.5 years. We were incredibly happy, then out of the blue... All because he was stressed with where he was in life and the growing up that was required after graduating.
I was extremely sad without him during that time, but I forced myself to focus on the fact this was not the end of the world. There were other guys out there, many of them nice. Sure none of them would be that guy (or in your case, that girl), but there would be someone. Eventually.
I suggest some journaling to reflect on the relationship, what worked, what went wrong, what you liked, what you didn't like, etc. Also brainstorm ways to carry on without her. Try to figure out more about yourself as a person, as an individual--especially if you never took the time before or during the relationship to do so.
Force yourself to go out with friends and family to do things. Isolating only makes it worse and allows depression to sneak in or completely take hold, so even though you may not feel like going out, go out.
Put yourself on a schedule. Up at __, eat breakfast, shower, get ready for the day, do whatever during, go to bed by __. If you're having difficulty, take it in baby steps of just getting up. Each day push for a little more to your routine.
Listen to happy music; watch happy (or action) movies. Avoid triggers like your guys' song or her favorite band or her favorite movie.
Eventually, you will likely have to go places that you two went as a couple--sooner if you live in a small town without much to do. Make sure to take someone with you, someone who is sure to cheer you up or joke around. Make new memories in these places.
Chin up! It does get better. Use this as an opportunity to learn to love yourself and being single. Being single does not mean you are alone! You have plenty of people in your life who will be there for you and will help if you but ask.
Best wishes!