When is self-doubt good?

Be confident young man! Believe in yourself! Show no fear! Have courage!

If anyone has been "guilty" of self-doubt in their life it is me. It has paralysed me at times to the point of quit, quit, quit.
No I can't be a writer. I lack descriptive powers and I'm too superficial. Too old now anyway.
No I can't be a nurse. I hate blood.
No I can't be a teacher. I lack the social skills and confidence. I don't like the limelight.
No I can't be a guitarist. I don't have the self-discipline.
No I can't become a manager. I lack a loud confident voice. And I can't even manage myself sometimes.
No I can't have those people over for dinner. They'll laugh at my cooking and presentation skill.

Excuses? Yeah maybe but these are the self-doubts I suffer. I guess the trick is to do it anyway. Just keep going despite the doubts.
No one likes someone with too much arrogance, lacking in all self-doubt. The happy medium would be nice. Some self-doubt is Ok I think. Stop us from over investing, biting off more than one can chew.
And another thing. Chronic self-doubt leads to chronic scepticism of others. Not trusting others can do it.