If you send her that letter i will be willing to bet good money that she will sit there and read it to all her friends and family and to OM. It will be further proof that "orange wont leave me alone. Why doesnt he get it. This is super creepy and boarder line stalkerish"
She will get off on having men fight for her and OM will get off on protecting her from you.
DONT SEND THE LETTER.
Get out of this mind set. The only way you can win this is by complete NC.
Fake it till you make it.
Take it day by day. Like your trying to give up sugar. Post here. " Made it 3 days with no contact" then make it a week. Then a month. You need it to break this addiction.