The devil will try to upset you by accusing you of being unworthy of the blessings that you have received. Simply remain cheerful and do your best to ignore the devil's nagging. If need be even laugh at the absurdity of the situation. Satan, the epitome of sin itself, accuses you of unworthiness! When the devil reminds you of your past, remind him of his future!
St. Teresa of Avila
from Today's Mass readings on ewtn
Now let me just add a disclaimer here. Saying one of my favorite saints is like saying one of my favorite chocolate candies. How does one choose? I love them all in their own way.