Doubt

There is healthy doubt and unhealthy doubt; healthy doubt being a form of discrimination, unhealthy being a chronic emotional habit of doubting oneself.

"Our thoughts are our traitors . . . "
Shakespeare

Doubt is the root of obsession-compulsions.
( Ex You lock the door, walk away, you get the thought "You didn't lock the door," doubt that you actually locked it, go back & check it again, repeat.)

"You tend to think that doubt is doubt of something, because when you feel into doubt it is always associated with something that seems doubtful. Thus, you tend to associate doubt itself with some thing itself that we are wondering about or feeling negative about, whereas actually doubt is a mechanism that has nothing to do with any object or thing to be doubted. Doubt is just another form or sign of the self-contraction. It is the self-contraction itself . . .
You should be able to identify doubt as self-contraction and not identify it with some object that is now doubtful to you simply because you are contracted."

"Self-contraction always takes the form of emotional dissociation."
- Adi Da, The Heart's Advantage

Conquering Doubt
The antidote to doubt is real "faith", in the sense of the ability to not give in to wrong doubt, becoming subject to thoughts of doubt.

This is done by a step-by-step practice (see here for the type of practice) of inner "stillness."