Seriously don’t worry about dating in your 40s/50s. If you can string a sentence together and iron a shirt you will be fine.
I was with my wife for 20 years and it was a real shock when she left. Lovely person and I was totally gutted to lose her. You worry about everything, but it passes. I have to be honest it took a long time for me to feel better but I do. I have a great life now, I have done lots of things I would not have done had I still be married. Is it better, I am not sure. I prefer to think of it as just different.
I felt really down, but I think we need to be careful to conflate that with mental illness. You are down because something bad has happened, that’t a normal reaction. It will pass.
Its hard I know but I think the biggest mistake I made with my ex was not being the person she loved. I fell apart and that was so not like me its unbelievable. I wish I had been able to at least give the impression I was not a mess and be the person she knew 🙂