Just found out that while I was with my ex bf he cheated on me with his intern I was suspicious of. What should I do?

There was a young girl he added on fb, he said he worked in his company but she didn't even work on the same floor. I always had a strange feeling. The other day I found out that he liked, commented all her pictures. He never liked or commented my pictures when we were together - he said his profile was for work. When we broke up he told everyone threat I cheated on him, etc which was farther from the truth. And discovering that he was so involved with this girl while we were together maybe explains why he reacted that way...