From first hand experience Elfy, it will stay with you a while, but really take this to heart. She is likely to have heard every word you said, the hearing normally goes last, and you would have given her the strength to be able to keep fighting rather than slipping away. What you did was amazing, so now as you pass the same area, tell yourself that, you did an amazing thing by being present when it really mattered to a very scared young girl. (This will help change the feeling of sadness when you pass this area)
Are you sure the boy passed away, they go staight to the one that needs the urgent care which is normally the one that is quiet.
Over the next few weeks, you need to be really kind to you Elfy, be patient with you, slow down and be kind to you. It will take time, and time needs time, but you will be able to cope with this given time too.
Well done Elfy