FWIW, one of my mom’s friends is widowed and is also a lively 70-something. She has a very full life and has as long as I’ve known her. I’m not even just talking about dating — though she does that, too. She is active, goes to yoga, enjoys her friends, takes language lessons, travels, and dates/has had relationships in her older age.
I think if you really look at your life as a waste you’ll have to continue to endure, talking to a therapist is a good first step. I imagine it’s not easy to leave a 50+ year marriage, even a dead one, because change is hard and uncomfortable… support may prove critical. But I do think there could be a lot of life left for you if you make some changes!