I am one who gets into a "funk" and is susceptible to feeling of melancholy more than the average person. I found this Daily Bread devotional written by David Roper to be helpful. Here is a snippet from it:
Sour moods create untold misery. Discontent, jealousy, bitterness, resentment, defensiveness, touchiness, and ingratitude are the dispositions that ruin our marriages, wither our children, alienate our friends, and embitter every life - including our own.
It's easy to defend our bad moods and to slide into deception and hypocrisy. But we must guard our hearts against such destructive attitudes. When they arise, we need to confess them, let them go, and experience God's forgiveness.
Don't let your bad attitude cause you to lose out while others enter into joy.
Resentment comes from looking at others; contentment comes from looking at God.
- Comrade for Christ