First I do recommend you read DR. If you can't afford it get it from the library.
Sounds like you didn't really make any changes after your first separation, you just went back to the normal routine and things eventually deteriorated again. This time you need to take a long hard look at yourself and identify things that need improvement. Then you work on them for yourself. Because if you don't, you will repeat the behavior and continue to have R problems no matter who you are with.
You said one of her complaints is that you live like roommates. Are you LD? Do you know why? That was an issue for me and I went to the doctor and discovered there was a medical reason for it.
Re: her letter, what specifically do you want to address? Why?
If you love somebody, set them free. http://tinyurl.com/2empx2g