After working on too many projects in totally different domains, using different languages, how can one get rid of the feeling as if all these years' efforts are nothing more than illusion?
I feel as if the only thing I know is how to pick up any new stuff fast, but ultimately it's not gonna last.
BUT, now that you know (intellectually) that it's a normal mental illusion, it can be disregarded. This doesn't make it stop happening, any more than knowing about the blind spot makes it stop being a blind spot. But, you know not to listen to it, like you know that something disappearing in your blind spot doesn't really mean it's not there.
In fact, I now use it as a metric of whether or not I'm learning enough new tech, or if I'm staying with what I'm comfortable in too much (which is a path towards obsolescence when you're a programmer). When I get feelings that I recognize as Imposter Syndrome, I know (intellectually) that it means I'm working on new knowledge, and that's a good thing. It doesn't make the feelings go away; learning more about the new thing will eventually help with that (temporarily). But intellectually I know I've worked on enough projects to not be an imposter, more or less by definition, and I know that the feelings of "I don't know what I'm doing maybe I'm not cut out for this" are normal, and not an accurate guide to whether or not I am able to do this. They are, of course, an indication that I need to keep working on learning whatever the new thing is.