I stopped worrying about being an impostor when I started writing about what I was learning. Confession that gave me confidence.
It's easy - or at least it has become easy - to write: Here is something I have learned a little more about. I didn't know it before. I haven't mastered it yet. You might know it already. Then again, you might also find this useful. Anyway, here it is. Would love to hear from you.
Writing like this throughout the years, I discovered that people didn't mind if I didn't know something. People were glad I wasn't promoting myself as some kind of expert. Even without people's validation, I liked myself as a learner, and I couldn't care less about being an expert.
I still sometimes get the momentary "Do I really know enough about this to talk about it?" when planning a presentation or starting a project. But most of my presentations and projects grow out of my blog posts, so (a) whoever invited me knows how I think already, and (b) I don't care about having all the answers, just about asking good questions. And writing things down, and sharing the ideas with others.
When you're not The Expert, you're not worried about being caught out or embarrassed by something you don't know. You don't get ossified into the few patterns you'd become good at. You can keep learning. You can make mistakes. Your ego isn't on the line. Your self-confidence isn't, either.
It's easier this way, and it's more fun too.