David, John the Baptist and Peter, along with many other Biblical heroes, struggled with various doubts about God and His ability or desire to help. God does not mind doubt as long as you are seeking answers from Him in the midst of it. Doubt can become sin if it leads you away from God to scepticism, to cynicism, then to hard-heartedness.
As you move closer to God, you will find the strength to trust Him and your faith will grow even stronger. I mean, the closer you are to God, the more you receive Heavenly resources and the stronger your faith grows. One of satan’s tactics is to get you to doubt God’s goodness. He tries to get you to forget all God has given you and to focus on what you do not have. If you are spending much of your time thinking about what you do not have, you may be slipping into unhealthy doubt.
God has given everyone plenty of evidence to believe in Him. Doubt comes when you fail to stop long enough to observe all the evidence. Once in a while, you should create time to reflect over God’s track record in your life. As you recall God’s track record, you will grow confident that He will work in your present situation as well.