Husband feels he is not going to last long

I have a weird question but it's concerning me. My husband had several strokes which turned out was being caused by a faulty valve replacement - the hospital where we live could not give us any answers and kept insisting the valve wasn't the cause. We took him to Mayo where they immediately diagnosed the valve problem and did emergency valve replacement with a mechanical heart. He is fine physically although very fatigued but he has severe cognitive issues and no self motivation which is so different than he was before this last stroke which was pretty massive. He keeps telling me that he feels very deeply that he won't last very long and definitely feels he won't reach 65. I keep asking him if it's because he feels the mechanical heart beating or if he's not feeling well or feeling depressed but he says that he's not worried or concerned and that he feels fine but he just has this inner feeling that he won't be with us very long. I've taken him to doctors and psychiatrists to make sure there isn't something going on (such as depression) but he's been given a clean bill of health with the exception of his cognitive issues. Has anybody else experienced this? He has been through so much but I have to admit his absolute conviction that he is going to die young is freaking me out - I don't want to just ignore his feelings and he doesn't dwell on it but just says it occasionally during normal conversation. I am afraid that if I just dismiss his feelings, he will stop talking to me about it