I am sorry for this set back. But you have come so far that two steps back is not going to stop progress.
I am not sure what all your letter entailed but I am sure it was the long version of these words I know it isn’t all my fault, I was there, I contributed, but so did he …it’s really unfair to blame me now – after all this time – horribly unfair. hence they say it all. Maybe simply put you should just say that sentence to him and leave it as is.