I suffer from impostor syndrome as well. I always tend to underplay my efforts. If I work hard on something , I always feel like, "oh this wasn't that hard and others could maybe finish this way faster than me. I'm just dumb"
I do not know of a way that solves this 100% but what generally works for me is reiterating the entire process in my mind a couple of times. Remembering the hardships that I faced and how I solved them, eases my worries a bit. Having the right friends is also very important. My friend circle earlier always used to make me feel uhh...'small' and lacking. This led to a lot of self-esteem issues. Fortunately, I made some good friends later. Talking to them always helps me. Hope this helps!