The bad thing about *sending* letters is they smack of pursuit even if you are attempting to "let go." You only let go by completely stopping that type of correspondence (and all relationship talk) and working on yourself.
Also, a relationship letter about all the cr@p only reminds them more of why they want to leave you. It only confirms it further in their mind.
Heck, leave the past behind, move forward. Pull your feet out of that muck. Quit rehashing it all. A very simple sorry about all the misunderstandings and then changing the subject would have been sufficient.
Stop the relationship talk. Work on creating "light" positive experiences and memories stop bringing up the "heavy" negative stuff. Time is ticking and you need to fast forward through the cr@p.
There is no arriving, ever. It is all a continual becoming.