Kureha, I too think you should tell your Psychologist too. She will know how to deal with the situation. You mentioned a knife. Can you do it without the knife? A knife is a weapon and you should not surprise any one with a knife. Hero or not, you will be accused of surprising someone with a weapon. Now, that's not pleasant.Please discuss it with your psychologist. And if you cannot, the letter seems like a good idea too. Just make sure she gets it and reads it before you do anything. You will be the hero for exposing that person.