I feel similarly.
I find that every time I want to share the knowledge in my head, someone else has done it "better"
The trick with that is:
1) It doesn't matter to your readers. In other words, the "better" version only matters to someone who comes across your page if they've already seen it.
2) If someone else's work is truly better, then it's fine to quote directly from it. For example I will never surpass the quality of Feynman's lectures, but there are still plenty of people who haven't been exposed to his work. The solution is to organize and cross-reference his explanations in ways that are more accessible for those who want to understand more on those topics. It's more important to explain something well than to come up with an original explanation.
I think ultimately it comes down to us worrying too much about being wrong, or somehow inadequate/ineloquent, or simply feeling like what we care about doesn't really matter to most people. But there are seven billion of us now. Even if what we say is irrelevant to the people in our environment, it may be a gem for someone else.
I'm going to try "writing to my younger self" or "writing to people of the future". It seems like a good method of boiling away passing fads and other irrelevancies.