I know there's a lot of advice about imposter syndrome that's basically just "feel your feelings" and I don't find that useful. So here's the one tactical thing that has actually helped me.
I keep a running doc called receipts.md. Any time something good happens at work: positive feedback, a metric that went up because of something I did, a problem I solved, a meeting where someone said "good point" and meant it. I write it down the same day. No editorializing, just the fact.
Why it works for the imposter syndrome brain specifically: The IS brain pattern-matches hard on negative events and explains away positive ones ("they were being nice," "I got lucky," "anyone could have done that"). The receipts doc forces you to log the positive raw data before your brain rewrites it.
When the spiral starts, I open the doc. Not to feel good about myself. Just to recalibrate what's actually true.
It's been 14 months. The doc has 83 entries. That's data.