Are you restricting fats?
Fat is pretty much necessary if you're restricting carbs (which you are, in practice, if you cut out gluten.) It sounds like you're trying to eat too little, and then eating "bad" foods when you inevitably get hungry. Fat is calorie dense and makes you feel fuller. You can pack cheese. 1500 calories worth of cheese is two bricks, and will definitely fit in your pack. I used to prefer sugar to fat, but eventually my preferences switched over, because I really liked the full feeling.
Since the beginning of September, I have been attempting a gluten-free diet. (I was tested and I'm not celiac, but eating wheat, and especially highly refined-flour foods like cookies, tends to make me bloated and give me diarrhea.) I also wanted to lose 5 to 10 pounds. I'm not overweight per se, but I possess a roll of belly fat that I (and my boyfriend!) would prefer to say goodbye too.
The first little while went well, and almost effortlessly. I was at the cottage with my family, exercising moderately (about 2 kilometers of swimming daily) and eating my mom's excellent-tasting cooking. After about one and a half weeks, I had lost 5 pounds, although I suspect a lot of it was water retention/bloating, since I had been eating wheat and various junk foods all summer.
Then school started, and with it my 16-hour days away from home, including one marathon session where I leave my parents' house at 5:00 am on Monday morning, sleep at a friend's house, and don't come back again until 11 pm on Tuesday, only to work 5:30 am to 4 pm at the pool the next day.
In short: my diet is quickly deteriorating and I have regained those 5 pounds. I find it next to impossible to stay gluten-free, since I have to be incredibly organized and pack everything from home, and inevitably it isn't quite enough for 16 hours. (I eat 3000 calories a day or more when not dieting. According to a metabolism study I participated in last year, this is actually how much I burn per day with the amount of exercise I get. If I eat much less, say less than 2000 for one day or less than 2,500 for several consecutive days, I get dizzy and weak when I exercise, which is really irritating.) I would probably be able to lose weight more easily if I exercised LESS, but this would a) kind of defeat the point, and b) be difficult because exercise is my main stress control method.
Willpower is a big issue, which is weird and annoying because usually it's not a big issue for me. Especially when I'm sleep deprived (nearly all the time), stressed, or bored to tears in my classes, I tend to comfort or reward myself with food, and nearly all my 'comfort foods' have wheat in them. I can resist to a degree if I have access to other reward/comforts, like sleep, or lots and lots of tea.
I've never really had to learn any willpower tricks for dieting, since I usually let my weight sit at its natural set-point. Does anyone have suggestions?