Is this cheating? Should I tell him?

In August last year, I started seeing my ex again, we spoke about why it was and both agreed that it was just fun and that we were lonely. The day before I went to uni we revealed we'd both started to really like each other but didn't want it to go anywhere. I then went to uni and during freshers week slept with someone else. But me and my ex continued talking, I never told him and around December we were in a relationship and still are. I just don't know if I should tell him this. I know that if I did he'd have a huge issue with it, as he was hurt when I got in a short relationship and slept with them when we weren't even talking. I just don't know what to do, it's not like it's always on my mind, but when I think about it, I do feel a bit guilty.

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