Wasn't coffee shops where you met, or at least where you primarily interacted, with her? Coffee shops are great places to go to work on your own yet being near people. They are great places to go with friends. They're not as good for meeting new people.

Especially if the location reminds you of the person you are trying to forget.

Did you ever try to look in to finding things specifically for people who are retired?

Art is nice, glad you're getting feedback. But it also gives you time to dwell in your thoughts sometimes. Same with walking - it's great but also won't occupy your mind too much. Plus you're likely to see couples which probably doesn't help to see as you're walking alone. Why not find a walking group? Or join an art class? Then you are doing things you enjoy and have an opportunity to interact with people!