How do you forgive yourself for being an idiotic fool?

You go to the root of the happening.
You think of what factors may have led to its realization. 
You acknowledge that none of them were in your control, even if they could have been in other circumstances.
And you realize you don't have anything to forgive yourself about, because you have done nothing willingly. 

Note: Make sure you bring to consciousness that it wasn't really your will for anything to happen or to do anything as it went.