I have to say, your rant is one of the better written, and more thoughtful ones I've read in awhile. When I go off on a rant, I usually have to restrain myself from blurting out a litany of four-letter words that would make even a hardened Drill Sgt. blush.
Your words remind me of an old Eagles song called "Already Gone". There's a line that says "So often times it happens, that we live our lives in chains, and we never even know we have the key". Like you, I have a love of animals, and the outdoors. I can no longer just throw on my overnight pack and head out to some new unseen spot in the wilderness. It's very frustrating. I once hiked across the Grand Canyon twice in one year, and now, the 100 yard walk to my mailbox leaves me fatigued most days.
You mentioned your parents, and your feeling of being chained to them. I understand they both have health problems, and although quitting smoking is a great thing, it can be a difficult thing to do, especially at first (my father was a bear for a month when he quit). You feel as though you need to be around to take care of your parents. This is not a bad thing. But, you also need to be able to make forward progress with your own life. Have you ever sat down and talked with your folks about you possibly striking out on your own, and following your own ambition and desires? What if they told you they would be OK, and could look out for each other? They sound very open minded and supportive, perhaps you need to hear them say it's OK to strike out on your own.
In regards to work. If you cannot physically work with animals in the field, maybe there is something you can do that would allow you to still be involved, but didn't require the physical abilities that you are not capable of right now. You could work for an animal rights/advocacy group, or maybe write articles and guest commentaries in animal rights/research publications. Have you ever considered going to school to become a Veterinarian?
My point is, at your age, you have many options still open to you despite your DIL. Lupus can, and will destroy the path that we have planned for ourselves and dreamed of, but that doesn't mean that there are no alternative paths. And sometimes, what we have lost to Lupus, can be replaced by people and things that are even better than what we lost.
Anyway, just some food for thought. Hang in there, and don't be too hard on yourself.