Doubt is an activity of the mind. While our minds busy themselves with thoughts, observations, anxieties, the Deeper Self is always there. It's the Vastness of Being that some call God. In its presence, the mind becomes quiet, even stillness itself.
You don't need to impose any conditions on your mind to become still. Allow yourself to empty. Release your identity, your cares, your anticipation and expectations. Let go and you will drop into the Vastness of your own Being, which is also the Vastness of All Being, one and the same, for eternity.