When I was bleeding heavily, I just felt extremely sleepy, life I couldn't stay awake any longer. There was a fuzziness around the outside of my eyesight, as though it was getting darker, like dusk turning to night and a gentle, swaying, sinking feeling like being in a warm, candlelit swimming pool. It was OK.
When I came round afterwards (and it had taken a long time for them to get me to wake up after the anaesthetic, so much so, I actually woke up with the consultant anaesthetist sitting on the bed stroking my cheek), I felt the darkness go in reverse. I was incredibly calm and just felt 'oh, he's nice'.
When I was being strangled, however, whilst I still saw the darkness closing in and felt calm about that and where my feet were dangling and hitting the wall, it was like I was kicking through water, I also found a reserve of sheer defiance - I lifted my head up, looked the cunt in his eyes and gasped 'Well, come on then'.
Which scared him so much that he dropped me and decided to give me a good kicking instead before running away. I didn't care and felt nothing of it until a couple of days later.
I believe that actually dying (not the process leading up to that point, but the actual act of dying) is probably just like that, but you continue to drift into the darkness until you know nothing more.