Ok, so what went wrong here?

I think he is in love with yu and he wants yu for himself, he felt betrayed wen yu started seeing someone without him knowing. As far as I can understand is yu two did gf and bf stuff(relationship stuff) which lead yu two to get attached but still denied what yu two felt deep down fo each other. And since he was monogamous that's a bummer, yu really got this guy for yo'self to the moment. So if yu love him, this your chance to tell him how yu feel bout him and the situation coz yu two won't do this forever yu will end up heartbreakin one another even though its a fwb thing. FYI fwb don't do all the things yu guys did like movies cook dinner ,breakfast....................buy birthday gifts fo each other. that was too much