I had the confidence that I was a good engineer most of the time and I have proved it more than once, but now during this period of my life, I suddenly feel weirdly inadequate...

You have to go through this plateau to become a maestro.

This is the real test whether you can come out as a nerd or a lost idiot.

Might be the Dunning-Kruger

Dunning Krueger vs impostor syndrome

The real dance with the devil for IT folks.

Do you have any kind of daily notes (A bit of stoicism) about your actions on your job and how it impacts in your life? What makes you happy? What practices and knowledge is desired by the market?

For me, it clarified that I was suffering with impostor syndrome at the time but then I use this as a fuel for continuous learning lol

Ah, And being a master of a given technology or practice is not the end but a ticket to the grand master league (next level).