Looking for a book with an unhealthy obsessed main character, where it is shown fully to the viewer how negatively their state of thinking impacts their life. (Not romance)

Not looking for romance, or the romatized version of obsession. The real deal, I want someone who's struggling. I want to see the hurt, the pain, and how this negatively affects the character. I'm looking for something that's descriptive, and doesn't take the main characters obsession as just a fact about them like I have [...]