Fight Doubt and Unbelief

[For Abraham, human reason for] hope being gone, hoped in faith that he should become the father of many nations, as he had been promised, so [numberless] shall your descendants be. . . . No unbelief or distrust made him waver (doubtingly question) concerning the promise of God, but he grew strong and was empowered by faith as he gave praise and glory to God. —Romans 4:18,20

God had promised Abraham an heir from his own body. Despite his advanced years Abraham was still standing in faith, believing what God had said would come to pass. He kept praising and giving glory to God. As he did so, he grew strong in faith.

It would be ridiculous for God to expect you to do something and not give you the ability to do it. Satan knows how dangerous you are with a heart full of faith, so he attacks you with doubt and unbelief.
Keep praising God and give Him glory. Faith will rise in your heart and you will overcome.