If that Devil gets any little hook in you, he will yank on it and pull you further down. It may seem all the worse when you are already lifted up. These are the most important times for prayer, especially when an important phone call gets no answer, an event looked forward to gets canceled, and someone gets upset with you over something silly all about the same time.
My favorite prayer at those low moments, is that of Divine Mercy..."Jesus, I trust in you!" These simple words are extremely powerful. They can be a tiny doorway back into a realm of peace. Remember them always and let them come from deep in your heart.