What symbol or metaphor could represent self-doubt?

[iamverytired]

This is for a school assignment, and I would appreciate it very much if you could answer quite soon!
I would prefer it if the symbol could be represented as a picture (something concrete).

[Calidore]

We don't do others' homework for them, sorry.

Hint: You seem to be suffering from some self-doubt yourself right now, so try looking inward for an answer. That may solve both problems.