Why are meals out so hard

I planned out exactly what I wanted to have days ago so I didn't need to worry, I have every calorie worked out. Then I hear "lets get dessert". I can't just sit and watch him have one on his own so I order the lowest cal dessert they had and told myself I'd only eat half, and then I end up eating the whole thing!

I'm still only 180 over my goal for today but i'm so so so mad at myself and already punishing myself with an extra long run tomorrow. Why can't I just go out not worry like a normal person!

I've been trying so hard not to purge, but I've just switched to exercise instead which actually makes me feel so much worse when I'm drained and aching.