Eat a great breakfast that morning, and maybe a small healthy snack before you leave, or bring some health foods to munch on if you feel like you might need something beyond a salad....maybe bring along a hard boiled egg to add to the salad.
And remind yourself when you look at that pizza why you are doing this. I suspect your 7 y.o. has something to do with it...being able to be more fully engaged with your kids is often a motivation. I find that helps me step away from the pizza, the cake, the cookies....whatever temptation is before me. With time and repeated success, it does get much easier and easier. Heck, I even decided I didn't want cake for my own birthday this year! And I had no regrets. You can do this.
A bad moment does not have to be a bad day, bad week, or a sign that you can't do this. It is a moment. Just that. Pause and go back to the person who really wants to be healthier and happier.