Jace had been growing incredibly fond of you lately. He always went out with you to fight demons, and you were the one person he trusted the most (aside from Alec, obviously). He also thought you were so beautiful. Literally perfect in his eyes.
But something happened. A bad accident while fighting a demon. Your saraph blade broke somehow... and you were put in immediately danger. It knocked you down, and just then it began raining.
You were left out in the rain in so much pain, you couldn't take it. You laid there, on the grass, bleeding out. You had a large puncture wound in your stomach... it hurt so much.
Jace reached your location as soon as he could. Tears were in his eyes, as he lifted your body up and held you tightly, crying a little bit.
At the Institution, he had you wrapped in bandages, sitting in the infirmary bed. He was drawing a rune onto your breast with his stele to heal your body.
"I'm sorry I wasn't there..."
Jace wasn't a man to show as much emotion he did today... but there's always room for change. Especially now, when you're dying and he's giving you healing runes.