As I wrote in my previous entry, we like to lie to ourselves because sometimes it's just easier. Let's take a look at the first lie: You tell yourself you can eat that cheeseburger and fries because you're drinking a diet soda.
First let's consider how many calories you need in a day. That's called your BMR and you can find out more about BMR and how to figure yours here. Let's say your BMR is 2500. That means you need 2500 calories per day to maintain your current weight and activity level.
Now let's consider that cheeseburger and fries. Obviously, where you get your burger is important, but let's just say you get it from Burger King. Go here to pick your toppings and see how many calories you will get from that. For example, a favorite of mine would be 710 calories. Fries would be 440 calories (and that's for a medium). Total: 1150 calories.
Now I grant you that a regular soda would be an additional 200-300 calories, and diet soda is definitely better on the calorie count. But seriously, can you afford to be eating almost half of your day's allowance of calories in a single fat-filled meal? Probably not.
We all love to splurge now and then on a burger and fries. Just remember to not splurge every day or even every week. You should probably limit these kind of high-calorie meals to once a month. Perhaps even choose a lighter alternative and then you won't have to lie to yourself to feel okay.