Being that close and involved in a death will always upset.
I had a similar situation when I was 19 and did as you had too. It was in vain and it affected me badly for a few weeks until I realised what I was being told - that I had done my best and that it gave him a chance but that it didnt' work out.
I suggest you seek counselling asap as it will help you get to grips with the feelings and put them in their proper place.
Talking about it here will also help a little, but not as much as a few hours with a trained trauma counsellor.
Loss is harder when it is so close. WHat you are feeling is perfectly normal and you will learn to understand and cope with it. If you want to, get me on MSN or PM or Skype me.
"A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'