Very Sad.
I am trying to put myself in your shoes to understand the pain/guilt that you must be going through right now. You must be really feeling depressed.
From what I can read, there was nothing that you did that could be deemed negligent. It is an 'accident' in the truest sense of the word. Hopefully the police enquiry will come to the same conclusion. I assume you have a decent driving record for a couple years - that will help your case.
Lean on your family and girlfriend - their support and ours will pull you back to normal. It is gonna take time though. Have you had a chance to talk to your friend's family?