I'm so very sorry. My heart hurts for you. I know there is nothing that I can say will ease your pain right now, but please know my heart is with you right now.
My 17 year old cousin passed away from cardiac arrest in January. She was walking out of a restaurant to use her phone. It's so hard to make any sense of these things when they happen to the very young.
My heart breaks for you and your family. I am so very sorry for your immense loss.