Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
I've been doing a lot of thinking and I just don't think that we are right for one another. This letter is long overdue and so much has happened in these past few weeks, all the arguments fighting makes our relationship very unpleasant.
What happened to make this the final outcome? It was a number of things, but in the end it was just not meant to be. I want to tell you that no matter what has happened, I will always value you as a friend and wish you nothing but the best in your future, and hope that you have a wonderful life.
I'll never forget you and how you have influenced me and my life. I'll always remember the amazing times we have had together and what we have taught one another.
Sincerely,