Saying the Jesus prayer is an act of faith. Lord Jesus Christ, have mercy on me a sinner. When we pray this, we are simply stating the truth of our poor spiritual state and asking for help. After all, not just nepsis, but even obeying the commandments, just getting through the day with a minimum amount of sin, is only by God's grace and mercy. We see in Romans 1, that when people entirely give up striving to worship God and exchange His glory for created things, it says that "God gave them over..." to their lust and their sin- What if He took away His mercy and decided to give us over to sin? What if He took away His mercy and famine or economic disaster hit the land? But don't we often take this mercy forgranted?
Praying this prayer throughout the day, as much as we remember, can help it sink in that we really can do nothing apart from Him. After all as the elder said, this is not a magical formula, it is a prayer, a conversation with God, a confession of who we are and who He is ... and aren't we all prone to forget how much we need Him if we don't remind ourselves constantly? Especially in the midst of all the daily cares and distractions of life don't we somehow keep thinking we are handling things fine ourselves, forgetting about the fact that He is continually with us and helping us, and if not for the mercy of God, what a mess we would be?