Well, it's probably impossible to stay entirely stress-free, calm, and relaxed...
but the first thing you have to do is decide for yourself that YOU are in control of your emotions, and you're just not going to let them get ahold of you. The holidays are stressful under the best of conditions, even for those folks who aren't dealing with a chronic illness, so you have to be even more aware of your limitations.
Lower your expectations. Things don't have to be PERFECT, even if that is somehow drilled into our heads. Folks should be coming to our house to see US..not our decorations, or judge what kinds of foods we're offering, or whether we're giving great gifts or not. I certainly am not a "Martha Stewart", nor are any of my friends (or family)...so if you come here, you'd know that I have 4 cats, lots of cat fur, and while my home isn't spotless, nobody's gotten sick from a meal I've served...at least not yet!
Shop by internet. Saves the crowds, the frantic running from store to store, the parking, the weather, the schlepping of bags. A few clicks (and a credit card) and you're done! Depending on where you shop, you can even get the stuff sent directly to the recipient, rather than having to deal with wrapping and mailing.
Consider not giving gifts at all! We haven't, for years. At least not directly. For the last 8 years we've written letters to our friends and family, reminding them of all they have (everyone has a roof over their heads, running water, food in the freezer) -- and telling them that instead of gifts, we've sent a fat check to Habitat for Humanity towards building a house for someone else. And we've asked them to do the same (rather than "gift" us). The first year was a bit awkward -- folks still brought presents. But now they know, and indeed they've done the same -- maybe not to Habitat, but to other charities of their choice, and told us about it. We're doing the same this year.
If you host the meal -- make it a pot-luck. Maybe you do the "big meat" -- turkey, ham, whatever --and have folks bring everything else. If you've got some folks who aren't good in the kitchen, have them bring dishes, or assign them the job of cleaning up. And don't be shy about asking for help! Generally folks really DO want to help, but we get so proprietary about our kitchens, we don't let "strangers" go in there, even when it's our sister, or mother, or best friend! Let them have at it! YOu can always put stuff away "correctly" the next day! You don't have to organize this really -- just suggest dishes, and let it happen. Generally folks will bring a nice assortment of stuff..but hey, if you end up with a table full of your turkey and all desserts - -won't that be a funny story for future years? And it's only one day...folks won't suffer from eating all desserts just that once!
Better yet, let someone ELSE host the meal, and YOU bring a favorite dish or two. And don't forget delis - -they've become a source of quality stuff, everything from mashed potatoes and gravy to some fancy entrees. Take the food home, plop it in your favorite dish, warm it in the oven (or chill it!)...and call it "home baked", not "home made"! and see how long it takes for folks to catch the distinction!
Families are challenging - -we all would like to think that we have perfect families...when in fact, most of the families *I* know..including my own..are pretty dysfunctional. We somehow think that when the holidays roll around, Uncle Bert won't be rude and sarcastic, Aunt Minnie will be sweet instead of bitter, Grandma won't be critical of our housekeeping, Grandpa won't grouse about not being able to smoke his smelly cigars in the house -- when these folks are like this for 364 days of the year, why would they change just because it's Thanksgiving?? They won't! And siblings -- why is it, when we get together with our now-adult siblings, we all start behaving like we're 9 years old, and the spoiled brats we were at age 9????
Anyway -- just decide...THIS year, I'm just going to "let it be". After all..what are the holidays really about, anyway? Impressing folks..or being thankful for the lives we have...even if they're complicated by some stupid disease or disorder. We're alive, and that's a good thing, so that's what I'm celebrating....
...I am not a doctor, nor health professional, and don't pretend to be one, here.....