Don't. The time you spent with that person and the memories you've made are nothing to be hurt by. Think about it, when you were dating there were two options stay together forever or break up. Maybe forever wouldn't have worked out, maybe the right person is out there waiting or maybe this heartbreak is way less painful compared to the heartbreak you might have had to face in the future with this person. It's extremely painful I can understand. But it'll be way easier if you see this person as a frog you met on the way to your Prince/ princess. Just an experience. :):) Good luck!
Do you really need to stop thinking about them completely? I mean if you try to block them all out it's going to be harder on you. Try thinking less rather than stop altogether:)
No way to stop it. Keep yourself busy, and as time goes by you'll find yourself thinking about it less and less. Hourly thoughts will turn to daily thought, daily to weekly, weekly to monthly, and so on so forth until it becomes just another memory.
I don't know your situation, but you might want to consider getting a new job.
hmmm, maybe you can't. do you know the 5 stages of grief? i bet you're in one of those phases.
try googling it :)