My mom brings out dessert every evening, it's testing my patience

I've been on a diet for three weeks, and I've even lost some nice weight, but I have a problem: My mom makes dessert every evening for sure. Even though she's on a diet too, she tells me "It won't hurt to have it just once," and puts it in front of me. I can't resist sweets; I really love them, but every time I eat them, I feel like all my efforts are wasted. I tell my mom, and she says, "I won't watch over you; you have to control yourself." Is she right? What should I do in this situation?