But much more surprising was the repetitive nature of my thinking. It was like my brain was stuck on a loop. I kept coming back to the same 4 or 5 thoughts/memories/plans, and kept replaying the same small slices of songs. I even came back several times to being shocked/annoyed by how repetitive my thinking was. If this reflects my normal mental processes, then I waste a TON of energy simply re-thinking thoughts over and over.

I have had things like this happen when trying to fall asleep. One thing I've found effective is a pad of paper or a voice recorder- when I find myself thinking "I need to get started on that new project" I leave myself a semi-detailed note to start on the new project, and then I don't have that thought again.

This sort of mind-dumping is probably better done as a warm-up to your meditation rather than part of your meditation itself, but I wouldn't know for sure.