Doubt is just another 'activity' of the mind. While our minds are busy 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 needn't impose any conditions on yourself to still your mind. Empty instead. Release your identity, your cares, anticipation and expectations. Drop into the Vastness of your own Being, which is the Vastness of All Being, accessible to all.