Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
I know how much I let you down. I also know how lucky I am that you can find a way to forgive me. I know that I can regain your trust. I know that I have it in me to be the _[man/woman]_ that you need me to be. I want to be that person, especially now that I know what life could be like without you.
When I think about losing you, I suddenly can't breathe. I wish I could go back in time and tell that foolish _[boy/girl]_ that I once was to stop. All I can do now is promise you I won't hurt you like that ever again. I know that it will take some time for you to believe in me. You can take all of the time you need. This second chance you have given me is worth waiting a lifetime for. I promise you that, everyday, I will work to prove myself worthy of being yours.