My inability to say no is making me cry. But what can I do about it?

I stupidly said yes to a job I should have run a mile from. This must stop

I'm working on something right now that involves reading a book I don't understand. It's on a topic so abstract and vast that I've cried several times trying to understand the first few chapters. The thing is, when I was offered this job, I knew I was temperamentally unsuited and too stupid for it. The conventional test for whether to agree to some future task is: would I want to do it if it were happening today? My answer would have been: Absolutely not - are you crazy?" So why didn't I say no?

Memoirist Leslie Jamison recently wrote in the New York Times about her Notebook of Noes", where she lists everything she turns down (speaking gigs, magazine commissions) and what that allows her to say yes to (more time with her daughter, her partner, her writing). It was inspiring: I cannot stress how little I am in demand, but even so, there are things I should decline (work I'm too dim for) to say yes to others (spying on garden bird drama).