Oh honey. I'm going to give you the same talk my Dad gave me and he is a doctor.
There is thing called acute numeritis (your number is up) you can have someone pass through the ER doors gravely injured or in an emergency and all the fates will align, they will live and go on another day. It wasn't their time.
But sometimes, sometimes people aren't meant to be "saved". No matter what you do, how long they are given life saving intervention- it is their time and they are going.
Death isn't a punishment. It isn't because you or anyone else did anything wrong or bad, it's not because they didn't deserve to live. It is just that they are due for the next stage in their life cycle and, if you believe it's a true cycle, they are just transitioning. After all we are stardust. All the atoms in me and you have been here to whole time. We never leave. We just change form. Change has to happen. The universe has to move.
You didn't do anything wrong. You did everything right. You went to go get someone who knew CPR. I do think you may need to talk with a therapist though just to help deal with it in a healthy way.
Hang in there bub you still have time.