The five hindrances are:
1. Sensory desire
Doubt can be the most debilitating of the hindrances. When there’s strong doubt, we can become unable to act at all. Some kinds of doubt are wholesome; we should not blindly accept what’s said until we are sure of it ourselves. But the kind of doubt that’s a hindrance is more like self-doubt or existential doubt; questions like “what’s the point of living?” and “why should I practice the Buddha’s teachings? (or other wholesome practices)”, or “it’s hopeless, I can’t change”. This sort of doubt can be oppressive.
To have doubt about mindfulness, of the value and importance of being mindful, borders on having doubt about the value of being present for life in general, because mindfulness and being present for life [are] the same thing.” – Gil Frondsdal (http://insightmeditationcenter.org/articles/FiveHindrances.pdf)
Investigation is one remedy for doubt. Look into the questions on your mind from a number of perspectives, narrowly and broadly. Consult with trusted friends or mentors. Don’t just wallow, pick up whatever mindfulness tools you have access to and apply them.
For me, another remedy is faith. Faith in what? Well, we can start with faith in the laws of karma: what we sow, we shall reap. Is this principle in doubt? If yes, we can investigate it more deeply: what evidence do we have one way or the other? Faith in the Buddha’s teaching can also be developed. Over the years, if we practice according to the Buddha Dharma, we will gradually come to understand that these guidelines for living are trustworthy and will protect us and show us the way toward freedom.
Are there other, more general principles that guide us? How about the Golden Rule (treat others as you would like to be treated)? Is that a reliable “guard-rail”? What are the principles that you have relied upon? Are they still worthy of your trust? What is the foundation you stand on? It may be something very simple, like truthfulness and kindness. When doubt is present, we may forget even something so basic.
As with all the hindrances, when confronted with doubt, the most important step is to recognize what’s happening – name the doubt, accept it as a normal part of our mental life, and work with it in as wholesome a way as we can.