How can one overcome the whispering doubts and fears that assail the supplicant after his initial certainty about the answer to his prayer, making him feel that his intention and reliance upon Allah are void?

You must be determined and put your trust in Allah, and occupy yourself with His remembrance instead of the devil's whisperings that deter you from supplicating and persisting in it. By increasing your faith in Allah, being certain of His promise, promptly responding to His commands, and frequently remembering Him, you will overcome the devil's whisperings; for Allah is near, answering the call of the supplicant when he calls upon Him.