When I read your post, Liz,I almost started crying.  I feel so sad for you.
This was a horrible, horrible, accident and you should NOT beat yourself up
over it.  I don't know why such things have to happen to us humans, but when
they do, we tend to relive the event and make ourselves sick with guilt.  I
had a similar thing happen to me.  My "boyfriend" killed my cat.
 
My kitty had been with me for 7 years and was my best friend.  When I moved
to Mexico I took her with me.  I met this crazy, jealous man and we moved in
together.  When we broke up, he took my cat away and killed her.  I felt
totally responsible for her death, since I should not have been involved
with such a guy in the first place.  I lived with remorse and guilt for
years and years.  I cried a thousand tears for her.  Finally I did something
that really helped .  I wrote a letter to her.  I said how sorry I was that
I had been irresponsible.  I told her how much her friendship had meant to
me and how much I missed her.  I told her that she had been instrumental in
me getting sober.  (I had been drunk most of the time in Mexico.) So, in
effect, her death woke me up and made me realize what a destructive path I
was on.  Her death had helped save my life!  I went down to the ocean and
read this letter out loud to her spirit.  I then burned the letter and
scattered the ashes in the sea.  I finally felt a closure that I had never
felt before.  I still grieve my Tiger, but, it is not nearly the guilt and
remorse that I used to feel.  I hope that this helps you.  I also hope that
you contact the "Rainbow Bridge Site" for animal companions that have"
crossed over".  My heart goes out to you.  If you need to "talk" please
contact my e-mail.
[Posted in FML issue 1809]