Help, this is making me breakdown....need advice?

Sounds like fwb wanted more to begin with, then when you went out with the other guy, it hurt your fwb, and when you came back he started being mean to you. In his own way I think the reason may be that he was/is trying to push you away so that HE isn't hurt again. He's distancing himself. In other words, you may not be able to repair the damage. But you definitely need to have a sit down with him and explain how you feel like you did here.