Nothing prepares you for your parent getting ill. No explanation about how entirely the proper order of your world is capsized conveys even a tiny bit of the enormity of it.
It can be simultaneously the worst and the best thing you will ever do, taking care of your parent when you're an adult. It alludes to the correct order of the universe, and is, in the crudest sense, some bit of payback providing your parent ever wiped your ass or did a nice goddam thing for you in your youth. But it is not pleasant. It is a nail in the coffin of your childhood, which, for many of us, carries on until this moment regardless of age. For most of us, we will always be on the receiving end, thinking of ourselves as somebody's child, until suddenly we're not. It will make you bloom and make you cry and make you angry and sad and scared.
Sometimes, it will simply make you practical. You will hold your parent's hand in your own. Their's will lack the authority it once had. The command. The strength that provided you the assurance that all would be well. And when you hold their hand, you will simply be as grateful as anyone has ever been about anything before that you have this one moment.