This is much like how I felt getting my PhD a couple of years ago, with a bit of imposter syndrome thrown in. Even though I know objectively the thing must be ok (as I passed examination, and I chose tough examiners), I still feel vaguely like I shouldn't have and that it is not much of a contribution.

It's like - I only passed through sheer hard work, bluster and a tiny bit of cleverness. On reflection, I suspect that's pretty much how anyone gets successful at anything.

One difference with your description is that I was only rarely exhilarated through the journey - usually when showing my work to those outside the field; I was much more often plagued by self-doubt.