Be careful when writing about mental disorders because unless you have personally experienced them, either through having one or through a close relative/friend, all the research in the world cannot prepare you for the hell they bring.
For those who have had or are still struggling with a disorder, reading about them can be triggering, especially if it’s written badly. But if you are going to incorporate mental disorders, curb the descriptions depending on your target audience.
For example; say Bob has paranoid schizophrenia. If the target audience is 14 years old, you wouldn’t write: His head spun around as he tried to figure out which voice was real. It was impossible for him to differentiate between the screaming demons that insisted that his mother had been abducted by the government and the nurse who reassured him that she was merely dead. Instead, you could write: Bob couldn’t remember what had happened to his mother. The voices in his head insisted that she’d been kidnapped but his nurse reminded him that she had died.
The point is, don’t write a description that could either trigger or mentally damage your audience. They turn to books to escape reality, not to be thrusted into a different version of it. Also when writing about grief and depression, it can be difficult to know where the line lies, so if in doubt, level it out. Unless the point of your novel is to be dark and twisty, don’t over describe the apathetic darkness that your character feels. Even if they are in the worst hell of their lives, play it down.
The exception to this would be if the depression or grief is the spark to your character’s actions. For example, in Moonlit Tragedy, Serena loses her will to live and acts as if consequences don’t exist. She jumps between starting fights against stronger characters to sitting in her room crying endlessly. The two extremes counteract each other but it becomes Serena’s secondary goal to get better. It enables her to go through a character arc.
I hope this made sense to you.