You don't have to fake happiness all the time because, you are right, it IS exhausting. Anyone with a chronic illness like this needs all the energy they can muster to lurch from day to day, and none of that energy can be wasted on making other people more "accepting" of you and your fake jollity.If you've been wearing the happy mask for a long time, then that is what people will expect. Once you take off the mask, they are going to have to adapt to the real you, the honest you, and if they can't cope with that, then you haven't lost much, to be honest.
Invite me to the next family visit. I can exchange barbs with the best of them. You might get disinherited but you'll damn sure enjoy the process.