If we consider honesty, do we imply that it will be a pure directness with feelings that are now present?
If yes, then take into an account that it is the last message you leave to that person.
If you want it to be a farewell, try to be objective. Like in every relationships there are rises and falls. You can consider both aspects.
But if you don't want to include anything good what happened to you, try to recall at least.
There are no good or bad letters as well as it is often hard to forgive.
Just listen to your heart and it will tell you everything.
Wish you all the best!
