I am no expert but for your career I would think you need an internal tape constantly playing in your head that says something like:
"I did the best that I could do. Nobody could expect more from me."
As for wondering if you could have done better, I'll bet that if you got those thoughts out of your head and on to paper (or a white board) they wouldn't play on your mind so much.
In my consulting practice I often teach businesses and their staff to create an opportunity log where they can track improvement opportunities, the impact of doing nothing different and a column for ideas with associated reduced impact.
It really helps put things into context and prioritize efforts versus trying to boil the ocean and feal guilty for not doing everything that is swimming around inside your head.
Maybe you could try something similar?
Also, one of my internal tapes that I had to create for myself was "Don't be silly. It is unreasonable to expect that I can prevent everything bad that might happen."
When you have the power to help others and the power to change things, you have to be careful to preserve a reasonable outlook.
Once you finally have tapes like that in your head, resolve not to argue with yourself
Just my uneducated two cents.