When the impostor syndrome hits me, I try to remember my achievements :
- I won a national coding contest when I was 18,
- I made and still maintain a complex app for 15 years, still actively used,
- I cannot count the number of languages I know; too many of them...

Not bragging here, btw. It's just important to actually enumerate your achievements.
If you get hit by the IS, just remember what you did 😉.