“Believe in yourself” is lousy advice.
I don’t mean to disrespect or discourage anyone, but I do suggest you expand your view beyond yourself.
Don’t believe in yourself. Believe in something bigger than you. Know that you are part of something bigger, something more powerful than you alone.
Are you part of the “world of science”? Are you part of the “world of sports”? Are you part of the group of people that fight climate change?
You don’t have to know them personally. You need to associate yourself with something more than you alone.
Try this; I guarantee you, you will see immediate results!