I feel for your sadness and heart break.
The worse pain is that of a lost love.
And the regret of not having done it all differently, is also a terrible feeling.
Here is my opinion on this situation:
1. If you send the letter, he might - psychologically - find things to fight and break up with the girl whom he is with, knowing that someone else is waiting for him.
And when he does, and goes back with you, three things might happen:
-you two will be happy forever
-it might not work out again, and he will blame you for having wanted him back.
-he might discover that he really loves the other woman, and break up with you, go back with her- another heart break will set in. And if the other woman doesn't want him back, he will also blame you for this.
2.)If you send the letter: He might not respond at all, which will make you feel even less loved by him.
I think you should try to meet other people, give those other men a chance, and try to either: forget him, or let destiny do its work.
What is yours, shall be yours, no matter what.
But once in a while, if you can find out how he is, and find out that he broke up with the other one, then, you can try again...first going out, then, letting love settle in for the both of you.
I know that it is easy to say and difficult to do, but try to keep the faith and the hope.
You might find another love, and discover that it was best that the past happened as it did. And you will be with your 'soulmate' forever.
We don't know, we never know. we only know that if we do things right, with a good heart, compassion and no selfishness, that happiness will be ours. That's all that us people know. Life is very mysterious, but it is also rewarding for those who have faith and hope.
I pray that you will find happiness,