A concept in my head that been rolling around a lot:
Hanahaki, but instead of it being triggered by unrequited love, it's triggered because all the love you have for a person turns inwards because you're too afraid to show it.
So it kills you, not because someone doesn't love you back, but because you don't let it out and all that love you have stored, that could grow into something beautiful, turns on you and turns your insides beautiful.
Love is growth, and without any place for it to grow outside, it grows in. If you confess, reciprocated or not, the disease goes away because it's no longer trapped. It gives self-destruction a new meaning.