It's one burger, and it's in your rear-view mirror. Now you have the choice to let it stay at "just one burger" or to let guilt drag you into other off-plan choices. You now get to choose whether you're going to keep climbing or fling yourself to the bottom of the canyon because you slipped a few yards down the cliff face.
Burgers aren't poison, they aren't heavenly manna, they're just...food. You had some of it that wasn't exactly conducive to weight loss, but it doesn't have to catapult you into a morass of guilt and hopelessness. It doesn't make YOU "bad" or "stupid" or "weak-willed" or all the other phrases with which we punish ourselves.
You want to make a lifetime change, I'm guessing, by the tone of your post. Well, life includes burgers sometimes. For most of us, the key to making a real, deep, meaningful life change isn't in never eating another burger, it's in being able to plan around them or even plan for them. Every time you sit down to eat, you're confronted with choices; the good news is that that gives you a chance to make smart choices multiple times a day. Instead of dwelling on one calorie-bomb of a choice, think about all the good ones you've made that have led you to your weight losses so far.
Dust yourself off, give yourself a hug, and learn from this. Learn that a misstep is not the same as a failure, that you can accommodate more calorie-dense food into a plan as long as it's a rare occasion, that every choice you make is a fresh one unfettered by remorse or hopelessness.
Please don't let yourself feel hopeless. You certainly aren't.
