This short video by Seth Godin is an instant classic for me.
Why do we have writer's block? What we actually have is a fear of being judged, a fear that our stuff won't be good enough, and so we say I don't have any ideas - I don't know what to say. No, you have plenty of ideas, but you're worried they're bad ones. The solution is simple - you just need more bad writing. If you do enough bad writing, it is inevitable that some good writing will slip through.
Making imposter syndrome go away is a difficult fool's errand, because the harder you try to reassure yourself and to rationalize, the worse it gets. The answer is to realize that you are an imposter and so am I. That when we feel like an imposter, it is a good thing, because it shows we are onto something. We're doing something that might not work. We're doing something that's worth sharing.