What now? We broke up and now I'm screwed..?

My (now) ex recently broke up with me and began seeing his ex (that he promised me he was over) and here I am all heartbroke again and I'm tired of feeling sucky. I will be moving a lot closer to him due to a job I recently got and until recently it seemed like a good idea. Should i completely avoid him or what? What is the best way to deal with this? Oh and it might help to add that I've known him since 6th grade (over 10 years) and his best friend is my cousin's best friend (who I'll be staying with until my academy starts at a prison. Don't worry I'll be working there. Anyway, how should I act around him?