You're Driving Me Crazy - Phan
You're Driving Me Mental sequel.
Phil is slowly going insane. Well, more 'insane' then he previously thought he was. Dan is definitely more 'insane' then before, making Phil, the supporting boyfriend, want to look after him. One day Phil snaps and walks out on Dan, moving in with his parents for a while. Feeling guilty, he goes back home, unaware of what he had caused.
You don't have to read the other book for this one to make any sense.