STOP beating yourself up right now! Choose to see it as less of a slip and more of a success = You ordered a small blizzard and thats it. You lived your life and made a decent, sensible choice. You could have done without it in the loosing phase, but you did have it. You didn't order a Large Blizzard, you didn't order a burger basket to go with it. You ordered a small blizzard for the cost of 345 calories. Now, plan the blizzard or calories into your day, be accountable for it. Recognize what happened and move on to have a great day tomorrow.
You have to live and make sensible choices after you loose the weight too. This phase is when you learn how to do it.