what seemed to be the symbol of her power in turn started eating her up

So, Yuyuko uses butterflies while her Shadow uses caterpillars.

Gelly_Breeze said:
So, Yuyuko uses butterflies while her Shadow uses caterpillars.

I think you mean Yuyuko controls the Philemon while her Shadow creates the caterpillars that would transform into Philemon.

Yuyuko despised/feared her own power over death, and her mind could not take the burden. Therefore she became like a fragile leaf, which then became food for the caterpillars that symbolize her power, the death that consumes all.
Of course, we know that she becomes a lot more cheerful and becomes a ghost, and starts to accept her power (though she still refuses to abuse it). This acceptance of her own power resulted in its manifestation becoming a butterfly - the ascended form of the caterpillar. Something noteworthy is that caterpillars become butterflies by eating a lot. Yuyuko's death caused by none other than her power resulted in that power achieving an ascending form...

...Okay, that's enough WMG for me today. I need sleep, and this pool needs Pixiv commentary and translations :P