1500 calories/day is not that much based on the active lifestyle you describe during the week. Don't let physical appearances fool you... you can be malnourished even if you look overweight. It's just a matter of consuming the right type of caloric foods if weight loss is your goal.
There is also a psychology to binging, which is that of deprivation vs. over-satiation. If you're depriving yourself during the week for the purposes of weight loss then you are far more likely to recoil on the weekend and eat way more. Dieting can be somewhat about reducing calories, but it's far more important to eat *healthy* calories. For example, if you binged on the weekend with greens there wouldn't be a problem.
The fact that you opt for cookies, which is a pleasure food, indicates that you might be in some kind of deprivation/satiation cycle. I would suggest allowing yourself some pleasure food during the week so that you don't take it in all at once later on. You also need to let yourself have more calories during the week if you're really so active.