Almost 30 years ago, after my defense (or defenCe -- for it was in England), I felt so ashamed of my dummy self and my research that I didn't even celebrate.
My complexes were amplified by the fact that in the country of my origin the standards for PhD dissertations in STEM were, those days, extraordinarily high -- a tradition established by Kolmogorov, Gelfand, Landau & Lifshits. and other titans. This contributed a lot to my misery.
It is only some 5 or 6 years down the road, and after more publishing, that inside my mind I pronounced myself PhD.
Years later, after I found that my PhD research was still being cited, I realised that my inferiority complexes were unfounded. Also, having been on several defense committees in several countries, I eventually came to peace with myself and gave to my thesis a fair grade: not outstanding but passable.
My case was not unusual. I knew a fellow who was so ashamed of this PhD research that he STOLE his thesis from the university library (it was before the digitisation era). He eventually became a well-cited scholar, but his impostor complexes were at that time even stronger than mine.
I conclude the story with Whitney Young's quote which, in my opinion, explains why we should never yield to the impostor syndrome:
“The truth is that there is nothing noble in being superior to somebody else. The only real nobility is in being superior to your former self.”
Keep on with your work. Climb your ladder. Do not measure yourself against others. Do what you can.