Ex-boyfriend hates when I flirt?

My ex boyfriend ( who broke up with ME!) got all pissy when he found out i was flirting with other guys. so he retaliated by flirting with other girls in front of me. it hurts a LOT to see him flirt with other girls especially since i still have feelings for him, and he knows it. and i don't EVER flirt in fron of him. he claims he has NO feelings for me, so he's just being controlling. any advice? i can't just ignore it, it hurts A LOT.