From "A Minute for Caregivers - When Every Day Feels Like Monday."

The Comfort of Gratitude

Resentment can lead even the best of hearts into poor judgment, wrath, and even self-destruction. For caregivers, the fight against bitterness remains perpetual. Maybe family and friends left us out to dry, a bad medical call, an employer fired us in a vulnerable moment, or a drunk driver who caused incalculable pain—there seems no end to the opportunities to hold a grudge. Some caregivers even resent themselves, while others shake their fists at God.
Regardless of the resentment’s object, the one who carries the hatred pays the highest price.
The only antidote to the poison of resentment is gratitude—the “…virtue from which all others spring.” Gratitude always leads to peace of mind, but bitterness only swindles serenity by deceitfully making us feel powerful in our wounded hearts. 
Thanksgiving is not simply a meal—nor even a holiday; it’s a way of life that refuses resentment’s tyrannical hold on our souls.
While incurred wounds are real and painful, they can only fester when we nurse them with the septic cloth of resentment. Thankfulness washes those wounds clean and allows them to heal. 

The soul that gives thanks can find comfort in everything; the soul that complains can find comfort in nothing.

—Hannah Whitall Smith