Help! I keep hooking up with this guy?

Ok so my bf's friendslept together the probleam is that I can't tell my bf. He would dump me yes I know I'm a slut say what ever I really don't care. I love my boyfriend but I hate his friend he alaways want to do it. It's good but not that good and when ever e get to gether it alwawys ends up the same I can't tell my bf it would brake his heart but it's bad cuz me and his friend don't even use Stuff... I tell him all the time to leave me alone he won't we just wind up doing it my bf best friend it's just really bad... And I try not to see his friend but I can't tell my bf... What do I do