I hope you got a more decent and honest explanation for the break-up than just the "it was a long time coming" and "I want to be able to make stupid mistakes" lines. If not, then I think she treated you very bad and very unfair. After three years of being together, you deserve something better than that sort of "explanation", which is, in reality, no explanation at all.

That said, there's probably nothing I can say to cheer you up, since there's only one thing that will heal this kind of wounds: time.

In my own experience, the best thing you can do now, is to avoid all contact with her (ignore her when you accidentally see her, remove her number from your phonebook, unfriend her on facebook, delete her e-mail and real adress etc etc.) and allow yourself to recover from this. Don't stay "friends" with her, it'll only make you feel even more miserable and will prevent you from "healing", since you'll keep false hope.

Do whatever the hell you want and that makes you feel a bit better during your recovery. Using the negative energy to get fit again by going to the gym seems like a good idea.

Speaking for myself, two to three months of no contact with the ex always did the trick for me when I was in your position. It was the period I needed to feel normal again. It's possible that you'll only need a few weeks, it's also possible you'll need 4 or 6 months, but you'll be fine.