until the end (but what will I do then?)
Astoria is fighting, you know she is. She's trying to hold on, for you, for Scorpius. All you want is to believe that she has a chance, and some days it's not that hard.
But she marches into a battle that will not end well, no matter what you do. This isn't like those Muggle diseases that they have spells and potions for. No, it's magic that has spread through her blood and made her so weak and pale that you barely see the vibrant woman you married anymore. Even if there were a way to fix it, tampering with something like blood is too risky and would probably kill her anyway.
The uncertainty doesn't help, either. Every time you look at Astoria, you wonder whether she has months, weeks, or just days left. The Healer you spoke to said a few months at most. You try to make whatever time you have count, but it's hard when you're having so much trouble accepting it.
You don't want to lose your wife, the love of your life, the one who showed you what love really is. You know Scorpius doesn't want to lose his mother, the one who used to sing him to sleep at night, the one who taught him how to make friends. Astoria doesn't want to have to say such a permanent goodbye to either of you, and both of you have done everything you could to try and postpone it, but the truth is that someday, sooner than you would like, Astoria will go to sleep and she won't wake up again.
You will stay by her side until the very end, choosing the moments you still have with her over the summer air, but this will not stop the inevitable. Astoria marches into a battle that will not end well, and eventually she will succumb. You want to be prepared for when that time comes, but no matter what you do, you just can't be.