It is the constant battle against sin which we fight daily - the refusal to allow the eye to wander, the mind to contemplate, the affection to run after anything which will draw us from Christ.
I feel myself getting bitter. Once I might have fed my desire by reflecting on all the wrongs I endure. But I realize now that bitterness is grumbling against God's goodness. And so (in my best moments and with God's help) I try to stop it before it grows.