ummm... letter sending is generally frowned upon...
Writing letters is ok, if you just write it... then throw it away...
If you do send... short and sweet...
"HONEYDO, I love you and wish we were back together, I have no idea why you won't do what I say or ask. What's wrong with you?"
or
"Loveydovey, I love you, I'm so sorry, I did everything wrong and wish you'd take me back, oh please, please, please..."
Those will generally not get you too far.
But something like:
"I just wanted to say that I've been thinking alot about how I have become someone that was not quite me, any more. I apologize for how you have felt about me or my behaviours. I appreciate I had the opportunity to be with you. Salutations."
You can pick amongst those...
Sorry if I'm making light. I understand that you are feeling emotional. None of this is ever good. But we learn, we grow, and things get better.