That sounds interesting. If it's a mental illness there are a few ways you could work things into the story. Research will be helpful and I bet there are a couple of days later in the month that will aid on that side. I don't know exactly how to write that but there is that Joker movie coming out soon, which might be a good reference.

I do think a protagonist can be the antagonist that gets in the way of what they want. You could even do a regular story for the protagonist profile and maybe with the antagonist one write a story if there are hints or manifestations of the illness when the character was younger. Depends on the illness and how it develops.