The Thirteenth Psalm is what is known as a Psalm of Lament.  These are writings where the psalmist goes from a cry for help, to describing a problem, to a petition for God’s help, to expressing confidence in God and finally to a praise for God.  This is not the exact order all the time but gives a general idea of how they flow.  There are several Psalms of Lament found in the Psalter (6, 7, 13, 17, 120, 130, 142).  In Psalm 13, David is lamenting the fact that he feels the Lord had abandoned him in light of persecution by his enemies.