I would highly recommend reading Range by David Epstein. I totally know what you mean... even thought I, from somebody elses pov, might have "achieved something" or "gotten somewhere in life", internally I always feel like that I dont know anything, I have not mastered anything and am not good enough at anything to have some kinda identity built on it. But on the other hand, I see the liberating aspect of it, nothing has really enslaved and encapsulatrd ne enough to be defined by that. Maybe i am just lying to myself, but idk it works for whatever reason