For let not that man suppose that he will receive anything from the Lord; he is a double-minded man, unstable in all his ways. NKJVA doubting heart should not expect answer to prayer from the Lord. How can one who doubts expect an answer? God may still answer, but when we do not fully believe that God will answer, how can we be surprised when He does not?
In my prayers by God's grace I will continually crush all doubt and I ask God to work in me as the master surgeon, cutting away all of those things which hinder my relationship with Him and produce doubt, which so often make my prayers weak and flimsy, hollow and without substance, 'safe' and 'small'.
I pray that God brings my heart to want what He wants for the reasons He wants it.