You really have more control than you think—or perhaps want to believe? You could have gotten water at Sonic, for example. You didn't have to get the shake. I know you said you didn't want anything with ice, but even water without ice will cool you down. And, really, if it were I, I would have ordered ice water, anyway, and dealt with the ice or let it melt before drinking, because it's a far better option than a shake.
You also didn't have to order the burger with your friends. There are so many times throughout the day when you can make good choices. You can go places that serve healthier fast food, and even at places that don't you can make healthier choices than you did. I know you wanted to celebrate, but you really can do that with your goals remaining top of mind.
I'm not attacking you. I'm trying to help. If you want this to work, you have to hold yourself accountable. You do have the power to eat better and lose weight, no matter your family situation. These things I mention had nothing to do with your family at all: you made these choices all on your own. And even at your grandparents you could have not eaten the pizza. You could have not eaten the chocolate cake. You really can succeed even in the face of obstacles. For example, people post here all the time about how they avoided the goodies in the break room at work. That's how they achieve their weight-loss goals: they step away from the pizza and cake.
Maybe these really were unusual days for you. Maybe you are eating well most of the time. If you are, that's great. But you can even eat better than you are at your grandparents, and at Sonic or other fast food places and so on.... I suspect you could even convince your grandmother to make more salad. But, if that really isn't an option, then you can still avoid the least healthy options for the few days you are away from home. If you feel you are being set up to fail, you very well may fail. In fact, the chances are you will. In my opinion, the way to succeed is to believe that you alone have the power when it comes to your weight loss. And, you do.