Boyfriend doesn’t know that I’m cheating with his best mate?

Well his best mate (friend) is my ex crush from university. We always had a thing but we suddenly lost contact. 10 years later I’m dating the boyfriend and it turns out they’re best friends. Needless to say I fucked the ex crush and we do it regularly behind his back and I’m not having sex with boyfriend much but I don’t want him to know. Advice please!!
Boyfriend doesn’t know that I’m cheating with his best mate?
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