I'm sorry, love.
I am sobbing as I write this. I don't have the guts to tell you the truth, that I have hidden this from you for months, and plan to hide it from you forever. I love you with all my heart, I am going to marry you. But I slipped up. All the rumors and gossip about me and that girl were true. I slept over at her house, we made-out, took our clothes off, and repeated this same scenario several times for a few months. I lied to you and everyone else. I cheated you. I have hated myself for months because of this. I played you, I lied to you, I made you look like a fool.
It isn't her fault, don't hate her like you do now. The reason I was so awful to you was because I had a secret and it was eating away at me. I hope you read this one day, love, and I hope you know that I wrote this. I hope that one day you can forgive me, and that I can start over. I love you with all of me, I am sorry.