Everybody feels like that. You just have to go for it, anyway.The problem with althist is there’s always more to read, always more context you can gain. I’m completely inconfident in what I know because I never feel like it’s enough.
The worst most badly researched story that is written is better than the best story that never gets started. Think of what you're writing now primarily as practice and learning the art.
Which isn't to say research isn't important (and also its fun to learn about history, I read history books even when I'm not writing anything) but ultimately you're never going to know everything, sometimes you just to have trust you know enough.
Like how much research do you think published authors do when writing historical fiction? Almost certainly less than you will.