Have you thought of caffeine instead of sugar to help keep you awake? Overdoing either one isn't great, but if you're concerned about calories, at least coffee has negligible calorie content and diet soda has nothing.
Remember that your midterms and stressful period WILL END. It's not going to last forever. It's not an excuse to eat everything in sight. Be gentle with yourself, sure, but being stressed out does not justify undoing all your hard work! Snack on popcorn... baby carrots... string cheese... yogurt... There are ways to get around the stress eating without driving yourself nuts!
As for the boy, congrats on the new relationship

Sounds like you might need him right now as a distraction from school. However, again, you can't use him as an excuse to fall off the wagon! If you two going out to eat all the time is an issue, make it a non-issue. Stop eating out so often. Tell him you need some sleep once every few days. You know?
You're in charge. You can get through this... And wouldn't you rather come out on the other side not having undone all your hard work? Good luck!