We often doubt because we've stopped looking at God and started looking at the circumstances around us. Like Peter walking on the water, if we keep our eyes on Jesus instead of looking at the waves, we will have a far better chance of living with a consistent faith!
But even when we doubt, we can take comfort in the verse: “I believe; help my unbelief!” (Mark 9:24) We can take even our doubt to the Lord and trust that He will help us with it!