kinda joking here... I don't have to read it...

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.