1) doubting yourself is what pushes you to learn new stuff, leave room for others to express and learn from them. I've had a fairly rapidly progressing career and I attribute a large part to that. Don't lose it.
2) at some point you start realizing that you are not the imposter, everyone is mediocre and just ignores it. You have been blessed with not being oblivious with your shortcomings, and even if you are doubting things you should not, even if it might seem like a curse, you're an improving human thanks to that.
3) when I freak out about whether I'm doing my job, at some point I usually realize that I am freaking out about whether I'm doing my job. When this happens, I tell myself: stop freaking out about whether you're doing your job and just do your f-ing job. It seems like you are very conscious about that so this might help
3.b) this is one of "the four agreements" which might help you: "always do your best". If you did your best, by definition there is nothing more you could have done. And therefore, no grudge you can hold unto yourself. After all, you did your best! What more can someone ask?
4) people are way, way more tolerant to failure than we give them credit for. I have been with companies that have very high standards and even there, screwing up is accepted as a fact of life as long as you learn from it. Just assume you're allowed to fail, with the current market, worst case scenario this will help you get another job in a better company
5) saw a tweet the other day of someone who changed their perspective from "I'm an imposter" to "of course I don't know what I'm doing, and I'm the freaking best con artist there is! I'm just faking all of it and no one has any idea,I'm so good at it!". Maybe that helps, at least it made me laugh.