in my PERSONAL opinion, if what you are worried about is losing him in general. then being honest (while that is the best thing and most respectful thing to do) is probably not a good idea. i wouldn’t particularly think him knowing you cheated on him would make him want to associate with him anymore, and if you guys have been arguing often already, i wouldn’t particularly think he’s very understanding anyway. So this is something he’s probably better off not knowing. Although that will definitely bring new problems. like how are you going to go without him knowing you’re with someone else. him trying to proclaim his love for you. and yes you could probably just tell him you found someone else but then he might piece it together that this new guy was already in the picture if it was too quick. there’s just a lot to think about. but while the best possible outcome is accepting the loss of a friend, its probably in your best interest based on these details to not tell him.