Do you remember the man who came to Jesus one day, asking Him to heal his son of an affliction that no one seemed able to solve? If anyone could heal him, he said, he thought Jesus could, but he also admitted that his faith wasn't very strong. He then added, "I do believe; help me overcome my unbelief!" (Mark 9:24). Let that be your prayer.
Have you ever asked yourself when doubts seem to come upon you? Elijah's doubts came when he was fearful and exhausted and convinced his life was a failure (see 1 Kings 19:1-10). Thomas' doubts came when he refused to listen to those who had seen Christ after His resurrection (see John 20:24-25). Other examples from the Bible could be mentioned -- but in each instance those involved had taken their eyes from God and put them on their circumstances.
Take time each day to be alone with God, through prayer and the reading of His Word, the Bible. In addition, spend time regularly with God's people in worship and in spiritual fellowship. The closer we are to God, the less chance Satan will have to fill our minds with doubt. The Bible says, "Submit yourselves, then, to God. Resist the devil, and he will flee from you" (James 4:7).
Doubt will either drive you away from God, or it will drive you closer to Him. Don't allow doubts to take root in your soul, but immediately turn them over to God and draw closer to Him.