My Husband Of 29 Years Has Decided He Wants To

My husband of 29 years has decided he wants to leave our marriage because he is not happy anymore. He feels like he has nothing, has been sexually active, drinking, missing work, getting in trouble at work for poor work performance. These are things that go completely against what he has always stood for. He was diagnosed about 10 years ago being bipolar. He takes Zoloft. He has always taken such pride in his work . He has always been reliable and wants to help everyone. His sexual promiscuity has involved paying for sex. These things are not him. How can I help. He tells me he is bad. He has even talked about suicide. He is living with a "friend" who is an alcoholic biker...not his typical friend.