Lots of love and hugs going your way, hang in there.
Regret is an ugly soul-sucking being :( It's hard but you can't feel guilty about not being "there". He passed peacefully and happy inside a loving home, on his own terms. And even when you're giving 110%, you would be upset that it wasn't 150% :) It's your nature. And who knows, if you would've gone a different route, he could've ended up stressed by the car ride or something else that you couldn't have foreseen and then would've been beating yourself up over.
I hear the same sort of things being spoken to me over and over, so I hope you can find some comfort in them :) We can't change what happened, we can only find peace in what good we did do, and look back at the happy memories!
you are in my thoughts *hugs*