A letter seems weak to me. She already doesn't respect you -- especially if she's running off to see another guy. A conversation may be more difficult but you've got to earn the respect back.
It's either that or go completely dark. Don't talk to her at all. You don't want to know because you don't care.
Me: 47, Ds 17-13, D final 6-11 http://tinyurl.com/yk4e2tz http://tiny.cc/thread2 http://tinyurl.com/ydtphqu http://tinyurl.com/thread4 http://tinyurl.com/3sm78k6 http://tinyurl.com/thread6
