Ok, so I'm having some trouble figuring out what to do:

1. I unlocked Uta's final heart and she is ready to ascend, she tells me to fill all of my hearts. The trouble is, I only have 20. I killed a few brothers - because I was told they might have my last heart - but nothing happened. Only Montgolfier, Tyrant, that drill guy, and Ironside remain. Am I doing something wrong to find my final heart?

2. I am nearing day 30, and since I did not kill the worm, that ends my game. I killed the Whale, but I am having trouble killing the worm - is it necessary? What are some tips to doing this?

So I can't seem to do the right thing to get my final heart. I'm so close to the end, I need some help! Advice please!

Bael

Stay cool, there's no punishment for not killing one of the monsters on day 30.
Also, don't worry about the last heart — build up a base for getting a lot of Color for now, you'll see, when you get the chance to get it.

Thanks! That did it! But now I have it, and I can't use it - do I still need to wait before breakthrough is usable?

Check the journal for precise breakthrough conditions — you need to fill all your palette and organs in order to succeed. Then either help a sister, or go to the adit and save yourself.