I’m feeling exactly the same way right now. TitsMcGhee gave great advice here at the end. I do think it is important to allow yourself that grieving period.
Besides that, though, I would say, get distracted. Hang out with friends and family, watch movies, get out and exercise, do something you’ve never done before (hobbies). And do not contact that person! Eliminate them from your life! Delete their email from your address book, their number from your cell phone. Put everything that reminds you of that person in a bag or a box and store it away, out of sight (out of sight out of mind, right?)
Also in the same post I mentioned before I talked about some visualization techniques if it’s really bad. I’ll repost them here:
1) Imagine you and this other person are each holding the end of the rope. Then imagine the rope being cut or burned so you are no longer connected by it.
2) You are looking down a long hallway. At the end of the hallway there is a door. This person you are trying to forget opens that door, walks through it, and closes it behind them. That’s it. Make sure that in your imagery they do not look back at you.
It may seem silly. It’s more difficult than it sounds. But it really does help.
If it’s really bad yo may want to consider talking to a therapist/shrink. Friends and family are great, but sometimes it’s good to get an outside, objective perspective on the situation.