The first principle is simple but extremely important. What matters in mental prayer is not so much what we do, as what God does in us.
[St. Therese of Lisieux] had a problem in her mental prayer: she used to fall asleep! … But she wasn’t overly upset by this weakness: I think how little children please their parents just as much when they are asleep as when they are awake; I think how doctors put patients to sleep in order to do operations. And finally, I think how “the Lord sees our weakness, he remembers that we are but dust” (Autobiography, manuscript A)
For the moment, what we need to grasp is that if, despite having good will, we are incapable of praying well, or producing any good sentiments or beautiful reflections, that should not make us sad. We should offer our poverty to the action of God. Then we will be making a prayer much more valuable than the kind that would leave us feeling self-satisfied. St. Francis de Sales use to pray, “Lord, I am nothing but a block of wood: set fire to it!”
—from Time for God by Fr. Jacques Philippe, Chapter 2: Part 3