Don't beat yourself to death! Do something about it.
In 1979 I shot out the window of a brand new pickup with a Ruger Mk 1, while going down the road at 65 mph. Stupid, Stupid, Stupid!
I felt like i should just sell my guns and quit. But, I got some good training, and more good training, and never ever forgot how easily I could have killed someone. I think of that incident everytime I pick up a firearm. I tell every new shooter that story, and then teach them the correct way to handle firearms.
I guess this is my way to atone for my careless act. If i can pass proper firearms handling along to newcomers, maybe I can save someone from feeling the way i did, or worse.
Anyway, my .02 worth. There isn't any excuse for being careless, but after it's over...Now What Are You Going To Do About It?