This ends the "Heart Dive" part of this story (don't worry, the story's not over yet).
If you got lost, what's happening is that Willow's fellow healer's joined together (into one consciousness) to dive deep into her heart's pain. They experience it as a visual manifestation of her pain... in this case, the future vision that Princess Celestia knew would become reality if Nightmare Moon wasn't stopped. A bit of that vision leaked to Winter Willow and has been slowly eating at her and becoming more dangerous over time (a malady untreated...)
It was too much for most of the ponies and they fell unconscious and dropped out of the link. Unable to maintain the link alone, Heartsong feared it was all over, but Rose Petal held on. Together they found Winter Willow's heart manifested as a diminutive and child-like version of herself.
In this comic, Rose and Heartsong are standing with her to give her strength and encouragement, but they can't actually do anything else. They have no effect on this world which is why ONLY Winter Willow casts light on the objects in the world. As Willow sees that she can stand against the darkness, she becomes stronger and braver. Rose and Heartsong stand with her for hours or days... time is unknowable, but their strength is faltering. They guide her with the words that Winter Willow herself uses to heal others; it's the truth of heart pain.
In my personal experience, things that hurt, old fears, agony, trauma... they never heal EVER. No matter how old you get. But they can become weaker and fade to the point that you remember them, but no longer feel them. Doing so requires that when you recall the pain, you don't throw it out of your mind. Grab it like a lightning bug in your hand and stare at at. Let it hurt, but don't look away. When you stop running, the pain begins to weaken over time.
That is what this story represents. It's Winter Willow; through her own pride and fear falling victim to the same weakness that she helps others overcome. She's so far gone after a time that she barely survived it, but as of this moment, the danger has passed. Her healing isn't complete, but it's finally begun and her Heart sensing has returned to full strength.
The ugly pony is a damaged and ugly fragment of Willow's heart.
The last scene is Willow at near fulls strength while Rose and Heartsong have both become very weak. Theyr'e not dead... Willow just sent them out of the link so they could finally rest because she was strong enough to stand on her own.