I have come to learn that "fault" and "guilt" are especially overrrated and overused words. One should be perfectly free to form his/her will, and then harm somebody purposefully. A rare situation indeed.
Can you try to write down what happened? Can you try to just tell it loud (just to yourself). I may bring the fact to light in a bearable manner. Since it is so triggering you may discharge a little emotional strenght from it. Maybe. You may try.

I wish you all the courage you need!