Why do i do this to myself!?

i do the atkins diet, less than 20 grams of carbs a day... i been doing in for 3 months and i love it. BUT i been getting these odd cravings which is making me want chocolate and sweets and i gave in about 3 days out of 30. when i give in i go overboard i dont just have one cupcake i have 4...which is not bad when i only do it 3 days out of 30 but then i get on the scale and i obsess over the numbers i feel terrible about myself, i literally feel sick, and i dont like what i see after one day off my diet. why do i do this to myself.!?