Where there is doubt there is unbelief, they go hand and hand. The bible tells us in Hebrews 11:6 But without faith it is impossible to please him: for he that cometh to God must believe that he is , and that he is a rewarder of them that diligently seek him.
God's word never changes, and you can stand on it forever. Mark 11:23 NLV I tell you the truth, if anyone says to this mountain, Go throw yourself into the sea and does not doubt in his heart but believes that what he says will happen it will be done for him. You have to believe when you ask that you receive.

Doubt in your mind that God will do what he said, comes from the devil, simply because he wants you to doubt God's mercy and grace, and anything else the Word says.
Find out in the word how to build faith in Jesus, you'll find it in Romans 10:17 search it out.

Father in the name of Jesus we pray for all those fighting the good fight of faith. Give them a hunger for Your word, and a desire for Your presence in their lives. We bind the devil over the minds In Jesus name. Amen