Plot- Jace has recently broken up with Clary and is miserable and upset and ends up just leaving the institute and OC finds him and wonders what is bothering him.
Later In the middle of a demon fight Jace collapses ill and OC tries to get him away from the battle seeing what happened.
Jace was in shock and upset , he had just recently broken up with Clary. He knew it was partly his fault, but it still hurt and he never thought that she would leave him. He was quite literally heart broken and didn't want to talk to Alec or any of the other Shadow Hunters, so he just left the institute, he wasn't going to come back unless somebody came and found him, or until he had calmed down, But for now he was quite upset.
He could hear some of them calling his name, but he just ran and had tears pouring down his face. He was miserable and didn't know if he could be happy again. Clary had always made him happy, but now he just felt like his heart had been torn out. He decided maybe that he would be best to not worry about relationships for awhile. He just felt like he would likely mess up any attempt to be happy again, and he was hurting too much.
Jace carried on running until he got somewhere he could be alone. He sat down and buried his face in his hands and sighed. He knew he had been reckless and stupid and lied about it too much, which had caused Clary to get mad at him and not be able to handle it anymore. He sort of doubted they would end up back together , and if he did he didn't think it was anytime soon. He wasn't even aware that anybody had noticed him.