http://www.wikihow.com/Write-Goodbye-Letters#Sample_Letters_sub
Goodbye! See ya! We've reached a crossroads, and I'm heading down a different path. Yes, it's time to part ways, and you want to be clear why that is, and you want to do it without the emotional give-and-take of a conversation. What to do? Write a goodbye letter! This article will give you some tips on how to do that.