My husband was killed by a drink driver in 2001. It's a horrible, horrible place to be when you lose somebody so close to you. Although the situation is slightly different what we are left to cope with is very similar. Like you, I had lots of questions in my mind as to if this or that had been different, would the outcome have been the same. I note that your girlfiend died in 2005, I 'coped' for over 2 years before the whole situation finally and truly hit, and I was offered the chance to see a counsellor. Please do try to do this, you will be talking to a stranger who won't judge you. They will understand any anger you feel, and will try to help you understand where it is all coming from, and ways to deal with it.
If it helps at all, I really thought that my life was over - as you have said all the plans you have for the future are taken away - I was 19 weeks pregnant, when he died, and I really believe that part of me died too, What I have to come realise is that yes, my old life died, but I now have a new life.
Unfortunately the experience will be with you for the rest of your life, but please get help - go to see your doctor, and remember although you have lost her and have to cope with all the pain that goes with it, you both had a loving, caring partner. I look back now and think that it was worth going through all the pain and having our time together, than to have never known him at all.
I really do wish you all the best.