This is the story of Jocelyn’s early life, as told to Clary, so remember — “you” in this story is Clary, listening.*
Though this was originally written as part of City of Glass, it was too long, explained too much, and had to be shortened and altered. While it’s fun to believe that this is how things were for Jocelyn, this excerpt has to be considered non-canon or alternate universe, so don’t be surprised if things in future Shadowhunters books contradict this version of events, or if it contradicts things in City of Glass.]
I got the idea when reading The Mortal Instruments:COLS when clary is going through Sebastian's/Johnathon's thing and she noticing a note that says 'to my love' but she shrugs it off. this is the back story to that note: The story of when and how Sebastian/Jonathon found and lost his only love and how he found and lost his humanity...