Sloth was among the so-called "seven deadly sins." (Also known as "capital vices" or "cardinal sins").
And the definition of sloth has changed. We hear sloth and -- since it's part of the American psyche to be Puritan -- think "lazy." And laziness might be a part of this sometimes, but that's not getting to the bottom of sloth.
At its core, the sin of sloth involves believing that we are beyond God's help, that God is not gracious to us, so that we despair of God's kindness.
Psalm 3.2 captures this attitude perfectly: "Many there be which say of my soul, There is no help for him in God."
Most of us often think that way. We imagine that there is no help, no comfort, no mercy in God available to us.
But verse 3 answers this despair: "But thou, O LORD, art a shield for me; my glory, and the lifter up of mine head." God shields us, protects us, and lifts us up. Even when we despair, He remains faithful.
This is why continued prayer is important. We should pray even when we don't feel like it, even when we imagine that God does not hear our prayers, even when tired, despairing, scared. We are not always faithful. But God is faithful. He is always there to be our help. Even when we imagine He won't.