St. Faustine describes failing greatly when trying to perfect a virtue. Although we should work hard to develop virtue, our progress is dependent on grace. If we do not cooperate with God, we will be frustrated and fail.
“When one day I resolved to practice a certain virtue, I lapsed into the vice opposed to that virtue ten times more frequently than on other days. In the evening, I was reflecting on why, today, I had lapsed so extraordinarily, and I heard the words: ‘You were counting too much on yourself and too little on Me.’ And I understood the cause of my lapse.”
Source: St. Faustina, Diary: Divine Mercy in My Soul