In contrast to my obligatory "Chicks are car guards™" response to these kinds of threads, I'll act my age for a change and give my personal opinion on the matter...
Same here. Started Scuba diving, which I've ALWAYS wanted to do. Started playing Skyrim...and oddly enough, after his breakup, my older bro is now also playing Skyrim non stop.
Being active helps a lot, whether it be gymming or hobbies or whatnot. Lying on your bed staring at the ceiling & trying to rationalise things in your mind or to try and sleep away the pain won't help one bit. We've all done this.
The awesome thing about keeping yourself busy physically, is that you feel a lot better about yourself and you're too exhausted to dwell on crap. Plus, girls do this thing where they make themselves hotter after a breakup, so why can't guys? I lost 15kgs in 4 months after mine just by gymming every day instead of going home and sulking and I feel fantastic. Helps the self esteem more than you can imagine...especially after the rejection.
As for the advice on here telling you to focus on the bad thoughts, I don't agree with that. It would cheapen what you had with her if you do that and I don't think it's right to hate the person when they don't foresee themselves being with you long term. Rather have it hurt now than end up with a divorce on your hands.
Unless she cheated on you with your best friend or something, there's no reason to despise her. Just accept it for what it is. I also don't believe that everything happens for a reason, the reality is, things just happen. Life isn't intentionally cruel or kind, it's merely indifferent.
The way I try and see it:
If you're feeling the urge to get back with her, focus on what made you unhappy. Don't fool yourself by regretting the day you met. The more it hurts, the more the experience was worth it. It beats having spent a year with someone you felt nothing for, this way you at least grow and mature.
If you're feeling worthless for being dumped, focus on the great times you had and how made her smile...and remember that you have that ability to make the next girl smile.
You are not worthless, she is not a bitch. You are merely incompatible.