Believe it or not, anon, I was once in your situation. The only difference was her name.
So let me tell you what's going to happen.
First, you're going to be a fucking mess. This will last a week to a month, depending on your friends. I recommend as much social activity with them as you can manage. Movies, video games, /tg/-approved activities, just hanging out. Minimize alone time.
Second, you're going to have a lot of feels. You're already experiencing that. How are the feels different from being a mess? Well, the reason you're a mess is because you're miserable. The feels are all the other shit. Missing her, wanting her, hating her, the what-ifs, the empty space, the things you think of and experience that you're used to sharing with her every day, but now you can't. It's awful. If you're a fitizen, pour it into your exercise. Pump up your tunes and shatter your records. If you don't like that kind of thing, at least start running. You need some physical activity to burn off the feels. At home, let yourself indulge. There's no shame in eating your feelings for a couple of weeks. (Ben & Jerry's and Taco Bell were my weaknesses.)
The next step is the rebound. DO NOT GIVE IN TO THIS IMPULSE. You will want to prove that you're over her as soon as possible, by finding someone else. THIS IS A TERRIBLE DECISION. You will regret it. WAIT. You've been with her for a long time, and at a point in your life that was supposed to be formative and discovery-based. You need time to figure out who you are without her. Rule of thumb is one month for every year you were together, plus one for good luck. That's eight months for you. MINIMUM. Do not engage any romance or sex during this period.
She left you. It happens. Better now than later, would have been nicer sooner.
You won't ever be over her completely. But eventually, you'll be able to go whole days without thinking of her at all. And that'll be progress.
It gets better, anon. For now, focus on you.