Shortly after I met my husband, about 30 years ago, I found out that he had often had sex with both men and women in the past. When he told me he was in love with me and wanted to get married, I asked him a million questions, mainly whether he was sure that he wanted to commit his life to a woman. I told him it was perfectly okay with me if he chose to be with men and we just became good friends. He swore that he only wanted to be with me and that he only had ever had romantic relationships with women. His attraction to men was purely sexual, he told me repeatedly. He persuaded me and we got married.
That was what he continued to tell me until four months ago, when -- without ever even having a close friendship with a man, let alone a romantic relationship -- he told me he wanted a divorce so he could find a man to marry. He also told me that I was the only person he had ever loved and that he still loved me and that he constantly sobs at the thought of leaving me.
For more than a year before telling me he wanted a divorce to find a man to marry, we had been in marriage counseling where, upon learning that he had an overwhelming desire, I had agreed that he could have gay "friends with benefits" and we could stay married as long as I was his primary commitment. (We had stopped having sex years ago after he told me he was impotent related to health issues; he was not.) Apparently, he had engaged in anonymous sex with hundreds of men during our marriage.
By that point, I was just holding out and hoping the man I had married and lived with happily for 23 years would return. He underwent a complete personality change about seven years ago. Why that occurred is unclear, but during that time period, he had multiple surgeries, had experienced problems with anesthesia and had been in an accident with a large truck. Somewhat before, he had been diagnosed with bipolar 2, although the medications worked so well that he lost more than 140 pounds, starting exercising regularly and got his business on track. He went off the drugs years ago over my objections but did okay for a while. More recently, he was diagnosed with adult ADHD.
He started attending gay support group meetings four months ago and now has made a decision that, just three weeks ago he said he would never do -- blowing up his marriage to pursue a gay life. Foolishly, I actually believed him. Both of us are quite open-minded, and I truly thought we could come up with a way to stay together -- even if we were living as close friends or siblings -- and each separately pursuing what was important to us. I also have a full life with work, friends, family and other interests -- although none of them involving other sexual relationships.
We are very open about still loving each other and having a strong bond. But he is almost pathologically dedicated to pursuing his "new life" even though it means not only leaving me but dozens of friends and family members -- including some children who consider him a surrogate grandfather. He says he doesn't care. (His lack of focus also has destroyed his business and has lost him many customers.) He told me he has no interest in interacting with women or straight men anymore -- only gay men. Yet, he is dating no one and is not leaving me for anyone or anything specific -- just because he believes he will be happy. I've explained that happiness is a choice and that he could do that anywhere if he decided to. Yet, he still argues that happiness is something you get from the outside.
The problem is the old "to a hammer, everything is a nail" problem. I believe his mental illness is playing a role in what he is doing but the gay groups apparently believe that every problem is related to the refusal to be your authentic self -- a gay man.  He completely buys the second explanation without much proof because it absolves him of responsibility. I feel as if those groups have helped destroy our marriage.
Exacerbating the issue is that, when he cuts ties with me and everyone associated with our life together, he will have no support whatsoever. If he ever was diagnosed with cancer, for example, he doesn't have a single person he could call to drive him to chemotherapy.
My life has been blown to smithereens, although I'm lucky that I have satisfying work with an at-home business, and close -- and supportive -- family and friends. I'm aware that when he leaves me, I no longer have any responsibility for him but can't flip a switch and stop caring. He constantly talks about remaining friends, but I'm not sure I can do that. That did not occur after my first, 10-year marriage. 
I would so appreciate any thoughts or advice for anyone who has been here.