In my late thirties I was struck with a mystery illness. It started with rashes, moderate fevers, and a slow but steady increase in the normal pains of life. By the time I was 37, the aches and pains were greatly amplified and the fevers reached as high as 105 degrees. I had seizures and the fevers brought delirium while the rashes started to mainly be on my face. I started getting sunburns and sun rashes after just minutes of exposure to natural light and with them came awful fevers. I kept on going to work until I was scheduled right out of my job.
It took over five years for it to get diagnosed as lupus.
As to how I'd have done things differently if I'd known about it, I'd have been really, really pushy with my doctors and stayed as pushy as I had to be until they came up with a diagnosis that made sense. I wouldn't have put up with all the repeated poking and prodding of my lady parts rather than my brain when I was having neurological symptoms, either.
I'd give up working at my day job before my day job got rid of me. I'd forgive myself for being sick right away instead of wasting my precious time on hating myself for weakness for a few years. I'd let myself enjoy what I still could a lot sooner, embracing my inner foodie and inner artist immediately rather than treating them like enemy slackers trying to corrupt me. I felt like if I was too sick to work, I didn't deserve any pleasure.
Before my illness struck, I was used to being physically strong. I enjoyed riding my bike, climbing cliffs, climbing trees, hiking, playing Frisbee, tickle fights, athletic sex, and all sorts of other physical recreation. It took a long time to get used to the idea that those things weren't for me anymore and the ways I had to prove it to myself were not pretty. If I had it to do over, I'd try to tell myself, hey, those things are in your past for now, take care of yourself and medical advances in the future might give them back to you.
I also wouldn't let people who said, "but you don't look sick," get in my head and wake up my self-hatreds and I'd tell them they don't look like a-holes, either, but they are.
You can still be sick or feeble and enjoy life. If ill, injured, and elderly people got appropriate medical care and pain control in this country, people wouldn't have to fear illness, disability, or old age nearly so much. When my fevers and pain are under control I can have a great time; I just had to learn new ways to enjoy myself.