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This is an ongoing struggle for me as well. I find that when I start to fall out of the groove, my natural response is to tighten up and try harder. This leads to going faster, which leads to tightening up and trying harder, and of course that’s all kind of self-defeating. I have to consciously relax and try to stay slow.
A couple other things that help me with this — stop looking at the fretboard, and mind my thumb. At some point I got in the bad habit of letting my thumb get flat along the neck, pointing at the headstock. My hand and arm are a lot happier if I can get it to hang out more perpendicular to the neck somewhere around the second fret. Figuring out all the geometry for my particular instrument and body seems to be a long-term project.