Oh trust me, your eating is much more important than your exercise--not that exercise isn't important either! I would have been at this weight years ago had I paid attention to what I ate and not just relied on exercise
Have you chosen an eating plan? If so, make sure to visit that part of the forum as there are accountability threads in each one

Does P90X have a food plan to go with it? It has you eating protein, right?
There are also apps and websites to help you track your food intake. Just the act of actually tracking it can be enough to limit what you eat.
Personally I never wrote anything down, but I do keep track of my calories in my head. Just doing that really forces me to think about what I'm eating.