I understand your anxiety - I have bouts of it myself. We are going 12/27-1/1, from what I understand the worst possible time in terms of crowds. Here is what I am doing. First, by planning you are ahead of most people. Second, the advice I keep hearing from people that were there during very busy times (late Dec., Easter) is to get there early in the morning. Parks do not get filled up and crazy until later in the day (11 or later). This one is going to be tough with a teen (which is the situation I am facing). The plan is to just let DD know - either you get up early or very few rides, your choice! Third, I think a mind set. I decided that we will do what we can, we will enjoy what is there, if we spend more time watching shows and parades then that is fine. I will try to avoid the long (2-3 hour) lines, but if need be (if kids want) we will do that too.
Even with all that, I am thinking - what did I get myself into.....
Good luck!