Give yourself permission to grieve is the best advise I have ever been given in that situation. If you are hurt feel hurt. If you are angry feel angry. You may very well obsess over getting back together but try not to act on those thoughts. It is normal to have them. Eventually with time and allowing yourself to grieve you will let him go and be ok. Acceptance it is called.

Talk about it until you are blue in the face to whoever will listen and not try to fix you. Post a lot here. Journal about it. Write it all out. Cry. Pray for the strength not to act on the obsessions if you are so inclined and for the strength to get through it.