Yesterday I spoke of the confessions of Jeremiah as useful sources of reflection when you are feeling despairing. Given all the pressure to be positive in our society, it is interesting how often in Scripture there are passages that honor the feelings of despair. In my book, Experiencing the Psalms I explored the place of rage in worship and the value of the psalms of lament for addressing the negative feelings we often experience as a pastor. As Calvin said, the psalms reflect an anatomy of the human soul. You can find there an expression for all the feelings that a human ever experiences.
Some of the psalms that you might find useful when you are depressed or despairing are: Psalm 6, 11, 13, 22, 38, 42, and 88. There are others but that would provide you a good beginning.
There are a couple of ways to reflect on the psalms that might prove helpful.
An easy one is to place your name in the psalm and sense how it feels to be expressing those feelings to God.
A second strategy is to read a verse and then paraphrase it to apply it to your own situation. What you are doing is allowing the psalm to provide a framework for your prayers.
An important aspect of all of these approaches to Scripture is to recognize that you are not alone in your feelings and to recognize that they are appropriate feelings to express in blunt terms to God. Feeling despair does not disqualify you as a person of faith. As it did with Jeremiah, it may be a painful step to deeper maturity.