Set out, in order, the circumstances that led to what happened, the other people, what made you think the way you thought and take the decision that led to it, and everything that made you who you are - the person who had a part in what happened, but who had no part in most of what made him himself. How much say did you have in choosing your genes, or your parents, or the circumstances of your childhood, or the information and knowledge you acquired? How much say did you have in being made into the person whose reactions are these? Write about the part the others played in what happened. Write about what you did not know at the time. And say honestly how large your own part in it was.
In people who blame themselves a report starts up inside: I ruined everything, immediately; I should have, and should have, and should have - even when they were not the ones actually responsible. If you are one of them, you can also write about the goodness in you that pushes you to take that responsible place in the world, rather than only blaming yourself. But finish the investigation, and all the circumstances around what happened, and give yourself your true share of it.
You don't have to acquit yourself of something you really did carry a large responsibility for. Only give your part in it its true size.