I too am in my 70's and was in a sex starved marriage. Even room mates can be deeply close friends. Sometimes they can even be friends with occasional benefits. It is very hard to seriously consider leaving someone who you have shared so much for many many decades.So it sounds like you guys are just roommates, so treat her like a roommate. Do your own thing and run into her occasionally on your way in or out. Start living as though you weren't married. Because you're really not.
What I decided was to live my own life and work toward healing myself. Then I decided that my marriage would either get fixed or I would end it. Luckily, my wife and I were helped by a great sex therapist and some very good marriage counselors and have worked through that phase of our relationship.
We now focus on the present and the future. We also remember the past that was pleasant. There is no need to dwell on the rough patches.