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Write the letter, if you feel you must, but don't send it.  You don't know if he's in a relationship now, and if he is, you risk ruining that relationship. 
 
I'm in a very similar boat right now, aside from the fact that I am happily married.  I would love to have closure with an old high school flame, but I know through mutual friends, that I would ruin the lives of many, including my own family, if I did something like this.
     
     
    
    
    
    
    
    
  
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