You sound exactly like me 10 years ago. My husband left for a week when he was 50, then came back. I had caught him in an online relationship and I said that if he would go to counselling, I wanted to try and work things out. At the time, I thought it was a midlife crisis AND, like you, have been concerned about his cognitive state (dementia in his family). He went to three sessions where he lied to the therapist, then he refused to go back. From age 50 to 61, I caught him in several other compromising online situations--he always denied he was doing anything and made me feel like I was paranoid and being distrustful of him. Long story short, in April I found him talking on the phone (to the same woman from 10 years ago), checked my phone records and could see he had been talking to her everyday for over a year. When confronted, he admitted he had never ended his relationship with her and that for the last 7 years it was more than online. He is gone, I am struggling to make sense of my complicity and role in this and it is hard. My advice to you, is let him go and don't waste 10 more years of your life.