We have a couple of family members who have anxiety issues - one of them is not unlike you, and needs to feel like the trip is "settled" or else she cannot relax at all.
She has learned to break things down into manageable "chunks" (so that she doesn't feel so overwhelmed, like she is trying to bail out the Titanic with a teaspoon) and she has also learned as part of that to make lists. The lists are good for her, because she can check things off, and it lets her see the progress she has made on "settling" that part of the trip. Here's a couple of examples of how she works...
First of all, she uses "Notes" in her iPhone, iPad and on her Mac via iCloud so that everything syncs up, and she always has her notes everywhere! That gives her a lot of comfort, because she worried about losing, or leaving her paper notes all the time. Because the notes (which are her lists) are in her devices, and they sync automatically, she knows they are always wherever she is. (And by using iCloud, she can even access them from *any* internet-connected computer in the world, just by going to
iCloud.com and signing in, which gives her a feeling of comfort and control, because she can always get to her lists!)
Next, we break the trip down into each day. So, for example, if we are planning a 10 day trip, then we start by "roughing out" each day - Day 1 is a travel day, Day 2 we are a Disney World, Day 3 we are at Disney World... Day 10 is travel home day.
Next, we take each day, and begin to fill it in with more detail. If we hit a snag (like your concern about HEA fireworks views for your Grandmother) then that goes onto the list for that day so that we remember to "fix" it. Each day has it's own list of things that we will address. There's also typically other lists, like a packing list (very important) and a list for things that don't fit anywhere else. Most folks don't need to go this far; they can do their planning in the My Disney Experience app, but she needs the reassurance of her lists, so that's what our family does.
In the "Notes" app, she uses a feature that Apple has provided that allows you to create a "list" with little bubbles or circles that you can tap on to indicate that you have done that item. There's a nice little check mark right there to reassure her that it's done and "settled", and the circles without the check marks are the things that she focuses on. This has been HUGE for her, because without that visual cue that she was, in fact, getting stuff done, she never really felt like she was making progress, and so her anxiety would ramp up accordingly.
When we are in WDW, we make sure that we can create "safe spaces" for her if need be. If she gets overwhelmed, or if she begins to feel too anxious, we can make a family "bubble" around her. If the bubble is not enough, then we know that we can go to First Aid for a little while until she is ready to continue on. And if she has to go back to the Resort room? Well, it's still OK, because we are still at Disney World! We can still have Mickey Waffles in the morning and we are still all together as a family! If the worst thing that happens on our trip is that we don't get to see the fireworks, it's OK. Believe me, I will trade you a trip to WDW without fireworks any day over sitting at home!
And that's the last thing that I want to stress. Our family always works to remember that if that (insert your worst fear about your WDW trip here) is the *worst* thing to happen on our trip, will we still want to go? Will we still have fun? Will we still make wonderful memories? As long as the answer is always "yes", then you will be ok. I promise!
I'm curious - do you find that once you get to WDW (either during your time there, or afterwards) that your fears were largely unfounded? Or do you look back on your trips and feel like they were all disasters? Can you find a common thread that binds the "successful" trips (for example, you let your family plan the trip, and you just showed up) vs. the "disaster trips" (you tried to overplan every moment of the trip and nothing was on time and everyone was grumpy) Ironically, our family has found that the less planning we do - the more spontaneous the trip is - the happier we are overall. That's difficult for our family member with anxiety disorder, because she needs to feel "settled" and prepared, and planning everything is part of that.
I know that these strategies might not work for you, and that's OK. It's just how one person with anxiety disorder copes. The thing to try and remember is that although no trip will ever be "perfect" (because it *can't* be, there are other humans involved!) you have amazing tools and resources available to you to try and resolve as many issues as possible before you even leave your driveway for Florida! One of those is right here, in this forum. We can't solve all your dilemmas - that's true, but we maybe can help give you some ideas or options, and at the very least, information, so that you know what to expect. And knowing what to expect - even if you can't control the situation - can help sometimes.
I'm sure that a lot of people have told you "Don't worry! It will be fine!" and for them... it is, but for you, it never can be, because you feel as though you don't have that option. I will never tell you that it will be "fine" - but I will encourage you to find ways to manage your anxiety, so that you don't have to fear the panic attacks and subsequent disassociation. For some people, that comes from certain concrete actions, and for others, it may be that medications are required. Regardless of how you arrive at a point where you feel more confident, please know that you are not alone in the world, and that the DISboards can be a great resource for you when it comes to your Disney trip planning.