Can someone become a narcissist & will they always lie, manipulate & cheat?

I have been married for 27 years with most of those year’s happily married to a good guy. He has been solid up until 4 years ago. Long story short it started with we had exotic sports cars that we both loved & participated in car events with other people that became friends. He started to be very active on social media talking to lots of people that he did not know (most being girls) with pictures of him & the car (none of us with the car). This started to make me feel bad & I told him that to which he said I was crazy & jealous. He started having affairs with multiple women. The first one was someone that was a friend of mine & the others he met via Facebook, Instagram etc. several of them knew he was married & had even seen us together or met me & didn’t give a shit! He spun the story that we we’re separated & he was divorcing me while telling me how much he loved me & that our problems are because I am so jealous & insecure. He even called my father to tell him I had mental issues & was lying. He now swears he is not doing anything “bad” & wants to stay married to me with the expectation that we just continue on as a happy couple like nothing happened. I can’t do that! I love him & have spent more than half my life with him but feel he will do it again. Car is gone but he still is active on social media. Please be kind. I am doing the best I can one day at a time and a mess. This sucks!!