Picky, you've witnessed something terrible. I am sorry. it's obviously triggered you looking back over your life and picking out the worst of the worst moments. if you force yourself you can see the good things, too (although that's not going to be very helpful right now). you need to talk this through with a professional, and it sounds like that will start today.
for what it's worth, I think in time your eldest child will get over this and possibly even forget (or at least it will be hazy), but that may depend on how you deal with the situation.
you're probably still in shock, but try to see today as the first step in moving forward. I wish you all the best xx