Why would a 52-year-old man cheat on every woman in his entire life being euphoric one moment and miserable the next?

Sounds like he may be Bi-polar does he also self medicate with drugs or alchohol? Tell him to go to a doctor and get tested if he refuses end the relationship. You should have enough respect for yourself to not let anyone treat you some way that you do not want to be treated or in a way that you would not treat them.