Forgive us our trespasses as we forgive those who trespass against us.
Lead us not into temptation.
Deliver us from evil.
The pivot petition, so to speak, is #4, when we move from petitions with "Thy" to petitions with "us." Or, as St. Thomas puts it, the first three petitions are for things that "cannot be had in their fullness except in heaven," while the fourth petition asks "for the requirements of this present life which are here obtainable in their fullness."
But what if we've already got our daily bread? What if our temporal wants were met yesterday, are met today, and (absent a cataclysm) will be met tomorrow?
Psychologically, I think, it means that we have to work at accepting the truths that it is only through God's providence that we have our temporal wants met and that it is presumption to assume they never (or even later today) will go unmet.
In addition to this well-known difficulty of being grateful for things you've never not had, though, I wonder if there isn't also a difficulty in taking the rest of the petitions seriously.
Petition #4 is the most practical of them all. Even people who don't believe in God and don't care about temptation want their daily bread. If the petition we should most fervently desire, on the natural level, is one we can't gin up much enthusiasm for, then how fervent will our prayer be that, say, God's Name be hallowed, or even that we be delivered from evil?
Gratitude is a virtue directed toward fulfilling our duty of thanking our benefactors, but it may also support the acts of religion, mercy, and hope required to pray with honest hearts as Jesus taught us.