I've found the single best measure of my stress and anxiety levels (as at their worst, my subjective perception can't be trusted) is HRV during sleep. My Garmi

I've found the single best measure of my stress and anxiety levels (as at their worst, my subjective perception can't be trusted) is HRV during sleep. My Garmin watch tracks it, and when I actively try to manage stress and anxiety during the day, it goes from red (for weeks) to immediately recovering into the green that night. I like a good dataset and the correlation with my stress (backed up by papers on causation) is at least a really strong thing for me. If I try something new, I can measure the impact on my HRV very clearly. When I feel I'm burning out, my HRV over the previous few weeks tells the tale.