Some people can eat whatever they want and others can't. Some people get clear after accutane and others don't. I was one of those who didn't --- even after 2 rounds of accutane. Now I'm on the regime below and it is keeping me clear. Diet can help. What matters is what you eat on a daily basis. Messing up once in awhile shouldn't cause too much problem. You're still very young so your hormones are going full strength...this may make it hard to get totally clear for awhile. Follow what your derm says regarding medications...even after finishing accutane. The reason for this is that even if you get clear your skin is still prone to acne and you want to keep the pores from clogging up again. The following diet will help by changing the stickiness of the sebum in your pores and by lowering inflammation which also clogs up pores.
Eat whole foods. This means apples instead of apple juice (which has been processed)...whole potatoes instead of french fries (fried in cheap oils). Eat whole grains such as oatmeal or cooked whole grain cereals instead of processed cereals from a box (full of sugars, etc.). Think before eating if the food is whole or not. This means you can eat vegetables, fruits, grains, meats and fowl as long as it hasn't been processed. Eat fresh whole baked chicken instead of processed chicken mcnuggets.
If you think wheat is causing a reaction then avoid it but substitute yams or potatoes or rice. Rice is fine...even white rice which has been eaten forever by Asian cultures. However they also add lots of fresh steamed vegetables to their rice.
Eat organic and wild foods when possible.
Eat extra virgin cold pressed olive oil in tiny amounts for cooking only. Eat whole foods that naturally have fats such as avocados, free-reange chicken, etc.
Avoid dairy...all kinds...including cheese. Dairy has natural hormones (cow is lactating) that can cause inflammation and hormone reactions in acne prone people.
Avoid sugary foods, added sugar and processed fats. Sugary foods cause insulin jumps that cause inflammation. Most fats (canola, sunflower, etc.) are made with solvents and cause sebum to get sticky and the face to be more oily.
Minimize alcohol. Causes inflammation.
The above diet will help your skin to calm down. Be patient. It takes time. You will get more clear as you get a bit older. Many people have raging hormones into their 20's and then things calm down.
Treat your skin very gently. No harsh scrubbing. While on accutane use a lotion developed for acne prone skin. I like DML lotion (can be ordered online). It was specifically developed for post surgery and acne patients. It was recommended by my derm who is an expert in adult acne.
Hope this helps! Don't be afraid to eat...just eat whole unprocessed foods in abundance with joy!