Oh man, this feeling is something that creeps in during even the most inspired moments. The notion that any praise is simply a courtesy given, some kind of banal etiquette extended by others. But yo! It's not.
A motto I fall back on (and have fallen back on for the past decade) is "this too shall pass". The story's associated to a king, at least the story I learned. During times of great sadness and loneliness, he'd remember that phrase. Surely enough, in time, things got better and he'd be totally partying, having an amazing time, surrounded by love and comfort and wine and all the finery. But... "this too shall pass". He also remembered in good times, they are temporary, that time shifts and moves everything.
Alternatively, your dissatisfaction with an illustration is your growth and doesn't necessarily mean you don't like what you do. It's just your inner potential is speaking really loudly over how cool your work is (and how well it's received), so you have a bit of dissonance with yourself. Keep going, keep illustrating. You'll build a proud body of work in no time.