Actually, I think the "no day but today" message of Rent is essentially existentialism repackaged. You can't put things off to tomorrow, you have to make tomorrow happen today. Very Camus, in a roundabout way. And for someone with a chronic illness, though nowhere as horrific as AIDS in the 90s, existentialism even recycled as "no day but today" is very true--there's no telling what will happen tomorrow. You can hope, and hope is a precious bluebird in the soul, but honestly, if today is a good day, you have to seize it, make it into something worthwhile. Otherwise, you waste what might be your best day for a long time. I don't think it's as nihlist as you're making it out to be--there is hope, but hope comes in small doses and you can't bet on it, so take the beauty in the moment and create something out of it.