How to get over someone?

Short version. I broke up with him, hooked up with one of my friends, he found out. Things were really bad, mentally and emotionally for him. We still live together. We try and get along and have a good friendship. We also work together, he started something with a chick at our work, she also goes to my school. He talks about her and they are always together at work. and he acts like he doesn't know me. Going through this I'm realizing I may have made a mistake. How do I get over him? Help me, I'm losing my mind!
How to get over someone?
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