Dear M:
The functional reason for thinking about the past is to make the future a better future: thinking about what didn’t work in the past, will help in planning what to do in the future. It is healthy and desirable therefore, to think about the past (and the future). Problem happens when we think beyond what is functional.
If we think about the past and it increases our anxiety that we do, and we miss on the present moment (not functioning well in the present time, even being inattentive), that is a bad thing for us and for the people around us (the inattentive part).
I ask myself when I think about the past, if I think for too long, that is, I ask myself: is there something I need to do differently today, or tomorrow? I then make what I call a mental note (or written as well) of something I need to do. And then, let go, best I can of further thinking.
anita