What I always feel like a character should do in that situation (technology permitting) is to turn on a tape recorder, fight the villain, and listen to what they have to say afterwards. And then try to figure out how to fix the problems the villain is pointing out instead of just feeling bad about themselves.

I guess that sort of works for this. You could write down what the voice in your head is saying, and then read it when you're not feeling terrible about yourself. And discuss it with other people and see what they think.

The problem with just trusting someone else is that unless you are already on your deathbed, and sometimes not even then, there is nothing you can say where their response will be "killing yourself would probably be a good idea". There is no correlation between their response and the truth, so asking them is worthless.