Don’t. Just describe the rapid changes of emotion and how exhausting they are. This is a case where most people are MORE deaf to metaphors than to a description, they’re so overused 
Maybe a nature metaphor, such as the change of seasons or weather. When I try to write it out, it comes off as trite, like, “cancer is as unpredictable as the weather”. What I like about the idea is how death is a part of nature.
Ups and downs is good.
Are maybe Gamut of Emotions.
That’s a word we don’t see much.
QtM for the win! Mmmmmm…cheese