Eating one healthy meal won't make you fit. Eating one bad meal won't make you fat. When you decide to eat something delicious make it a choice not a cheat.
Actually, varying your calororic intake is a technique for weight loss by tricking your metabolism. Google some key words like,
How to Trick Your Metabolism by Varying Calories - Calorie Shifting. Something like that.
You aren't a failure. I'm not sure you want to increase it by that many calories, but read up and see what they are talking about. I am thin, and my calorie intake varies.