Everyone experiences self-doubt. Everyone.
“If a person — any human being — is told often enough, “You are nothing. You are nothing. You account for nothing. You count for nothing. You are less than a human being. I have no visibility of you”, the person finally begins to believe it.”
—Maya Angelou
Angelou's words apply to external lies you are told.
Every time you say terrible things about yourself, you begin to believe your own lies.
My self-flagellation favorites:
I can't do anything!
Tracy, you're an idiot!
Look at the mess you've made!
What negative self-perceptions are holding you back?