Should I call it a day?

My partner of 16 years has started to cause me great anguish. Over the last couple of years (after turning forty!) he has changed, and I am no no longer sure if he's the man I've known and loved for all these years. He bought himself a new sports car, started courting attention from younger women who flirt with him (which he denies, but which I've watched him do on many occasions), and after our last big argument about his behaviour, he's started referring to me as his girlfriend rather than his partner or his other half. I've noticed that on Facebook, he's started posting happy birthday or congratulations, etc, message to mutual friends and signing them in his own name only instead of from us. I feel that he is already down-grading our relationship. He denies that his behaviour is anything different from what it used to be, but his words don't match his actions. It's breaking my heart ... I love him but I'm considering leaving him before it gets any worse, as I can't face a slow and painful breakdown. What to do?