Doubt means there are two opposite beliefs inside you. One is your own, one is from the environment. The one from the environment is usually the loudest.

To release doubt, you need to pay attention to one belief at the time. Ask your both beliefs to agree which one wants first. Tell the other belief that they will not be forgotten, that their turn is next. Usually the loudest will want attention first. When you have given the first belief enough attention, it will go away. Then the doubt is effectively gone because there are no longer two beliefs that compete for Attention : there is only one left. And that belief is your own belief.

I should add that when in doubt, there are two opposite beliefs and you want them BOTH.

The opposite of doubt is choice. (de-cision: cut off all other options)