Giving in to one's temptation and eating fast food goes against the goal of abstaining from it, so someone can't be happy about doing that is not really successful in following through with their will power.
Consider the staetement "I have no will power because I swore off fast food this month and today I had McDonalds. ". Based on this statement, is "I feel happy about succumbing to my will power and eating McDonalds today when I swore off fast food this month" a true statement? Yes/no/unknown