One point of clarification still needed *SPOILERS*
I am of the opinion that it was all Henry going through "purgatory," as if his life (the alternate dream "reality" he creates) was literally flashing before him in a moment. This is very similar to the plot of Dead Man and Jacob's Ladder, particularly the latter one. He constructs an alternate life while his soul or consciousness is deciding whether he is to live or die. He decides it's better to die having lost everything and everyone he loves.
One thing I am still confused on though is why Henry felt responsible for killing his girlfriend and parents. Was it because it was his car that had the front tire blow? That seems like an odd reason to blame yourself, seeing as 99% of the time that is a complete accident that cannot be avoided. That would be like blaming yourself for walking people down an icy street and having one of them slip and hurt themself. Is there something else I'm missing?