The theme reminds me somewhat of Loop Hero - the world is gone, but what you see exists now.
It's a very impressive technical feat that you've managed to make all of this solo in jam time without an engine.
However, the playing experience is a bit rough as you don't really have much control over what tiles you're going to reveal, only what tiles you're going to put effort into keeping. And in that case, the black void does take away more from you than you can feasibly keep together, like a tide sweeping away a sand castle.
I would suggest trying out changing what your light reveals (or remembers identity) based on what you're standing on and develop from there. Then, you can try to keep your life crystal patch alive instead of having it die out slowly but surely. Or, for joined together patches of terrain to shrink to the darkness more slowly.
It just needs some tweaks, is all.