You asked HOW to explain to bf that his flirting hurts you.
As a 43 y.o. male, let me rish sounding like Daddy. WAKE UP !! You DID explain. He totally ignored you.
Drop him, or put up with a life of pain.
As a predatory, human male, I can't get the points across which you don't want to hear, but I am a guy, and older. Life ahead is a trainwreck with this guy.
Feel good about it, but dump him.
Express yourself, release your anxiety, but dump him.
'Help him understand' but dump him.