The House of Leaves by Mark Z. Danielewski. It wasn't scary per say but I had an interesting experience where I had a manic episode reading it, barely slept, and got absolutely obsessed with the idea of it as I read it.
10/10 loved the immersion aspect.
Oh, sounds a little bit like me.
I am genderqueer/nonbinary. Specifically, my gender identity tends to fluctuate between agender and female. I am "comfortable living life as a woman", but I often look at more androgynous folk and wonder: "Could that've been me, if I made a few different life choices?"
I can't confirm what your identity is for sure. But I can say that there's at least one other big person (I'm rather overweight, which tends to affect my dysphoria because there's no hiding these curves 🤷) that experiences something similar, if you want to explore looking up nonbinary identity.