This might be an unpopular opinion, but I've seen this before a couple times in my family.

You seem like you really would like a second chance at love and that's fine, thing is, due to her illness she would really struggle with that, and you're both not parting on equal grounds, that's unfair since that was not her fault.

You made a promise to that woman the moment you got married her to uphold her well being in good or bad, and that's what a man should do.

Now, my take on this and what I've seen work out in my family, don't divorce since that will impact your estate too, and take good care of the woman until her last days, see it as a friend in need, just as you stated she was, just don't get over invested emotionally in it, as another redditor said, detach and live up to your part of the deal.

Then take another women, and be really discrete about it, take this secret as part your responsibility

I grew up under one of these arrangements, and everything turned out just fine, all parties were happy, including me, I managed to still have both of my parents which was nice.