An apology.

The following is a letter I've written as an apology to a woman I've wronged. Our names have been changed. Any advice is welcome. I know plenty of you will leave hateful comments, and I deserve them, but do so knowing I feel terrible already.
Here it is:
Dear Kate,
I think a part of me always knew, somehow, the time would come when I'd write this letter. I've fought against it, because it will hurt everyone, and because it may ruin the relationship I've been fighting so hard to keep. I've convinced myself a thousand times that it can't possibly do any good. But that nagging desire for total honesty (that I've been repressing in Sam's name) has finally spilled over and turned into need. So I'd like to sincerely say that I am sorry, for everything you're about to read. For every one of my actions, that have undoubtably hurt you.
I met Sam in late October of 2011, having just moved into the apartment with my boyfriend. By mid November Sam made his first advance, which I refused. But before December struck, I found myself headlong into an affair. At the time I knew only that "he was talking to some girl in Mexico", and that was only because George had told me. I'd asked Sam about you, wondering the extent of his relationship with you, and never getting a full answer. That is, until Christmas came around, when he told me his girlfriend was coming to visit. When he called Christmas <a href="">Eve</a>, to tell me he was falling in love with two women. To ask if I'd want him to love me. I said yes. I realized quite suddenly how hard I had fallen for him.
While I had a hollowing sense of guilt, both because of you, and because of my boyfriend, at the time I also somehow believed myself when I said that what we were doing was inevitable. That it was an un-avoidable collision of fate. I loved him, with all my heart. And love is blind. It may seem trivial, but I want you to know, I would never have suggested that threesome. I would never have granted it. The disgrace I felt over what hurt I was already causing you would never have allowed me to disrespect you that way.
I did, however, believe that given time, he would leave you, as I had left my boyfriend. And so I waited. Over and over again, one of us would say we couldn't do it anymore. Over and over again, we'd fight it, just to find ourselves in each other's arms again. You'd come to town, sometimes he'd tell me, sometimes he wouldn't, but I always knew. He'd tell me he couldn't chose between us. He'd tell me it hurt him as much as it hurt me. He'd tell me he loved us both. We would break up, and a day later be back where we started. I'd hear him argue with you, or he'd tell me you had left him. He once left me sitting at a bar, alone, staring at his half finished glass of beer after rushing off to make things better with you.
And that was the way it went. For around three and a half years. I can't begin to count how many tears I shed during those years. How often I felt guilty to the point of distraction. How feeling second best crushed me, destroyed my sense of self and left me unable to think clearly. I can't tell you how badly I wanted it to simply be over, one way or another, but felt tethered by how much I loved him. I became a person I didn't recognize.
Before Sam, I would never have been "the other woman", having been cheated on by boyfriends in the past, and knowing the damage it can do. Before Sam, I was a strong, independent woman who would have laughed if you told me the things I'd allow, and the things I would do to stay in his favor. I felt myself becoming a shadow, competing with you, walking on eggshells to prove to him that I could be what he wanted. All the while being truly ashamed of myself; between pangs of remorse, I felt a green bitterness aimed at a woman who didn't deserve to be treated the way we were treating you. I felt envy any time I thought of you (which was quite often). I was jealous of your career, and how it could offer him a chance to advance his. I was jealous of your beauty, and intelligence. Your success, and your flawless class. I felt ridiculous, a failed tattoo artist, a house painter with bruised knees and chipped fingernails. And I hated you for it. And I hated myself even more.
Until you wrote me that letter. I was shocked to receive it. Shocked by what it said. Shocked that it caused Sam (for the first time) to say anything bad about you. I panicked. And in the end let him convince me that I shouldn't respond. But I also felt relieved that the two of you were finally over. At first it seemed as if you might get back together. I made it very clear that I would no longer be a part of his life if it happened. It didn't. We went our rounds about his continued communication with you. But in the end, I believed there was no harm in a long distance, platonic friendship between the two of you.
That's what he told me was happening. And despite everything I knew about him, all the lies I'd witnessed him tell you, I trusted him. When I saw that you had liked the photo of his cookbook on my Facebook page, I actually believed you were offering an olive branch. That the two of you had become comfortable with each others new relationships. I believed that I was his girlfriend, and that he was happy with me. As I said, love is blind. And I wanted so much to simply be with him, after so many years of drama. 
That is until a week ago, when I found out the truth. That he had been begging you to come back to him. That while I was trying to build a future with him, he was still clinging to his past with you. Telling you that he would have nothing to do with me, if only you'd be a part of his life again. That he is lost without you, and that you are the love of his life. He called you Monk, I pet name that used to make me feel so special and loved. My heart was shattered.
He tells me now, that he'll find a way to make it better. That he said those things to you, only because he wants you to be friendly with him, and not because he wants you back. He tells me he won't do it again. My response is that it sounds precisely like the things he was saying to you.
The truth is; I told him, if he doesn't want to lose me, HE would write you this letter. HE would tell you the absolute truth from beginning to finish. That it was the only way I could know he was ready to let go of his relationship with you, and to move on with me. He refused. Once again proving he'd rather lose me than hurt you. 
So I guess I'm writing this letter so that finally, we both know the truth. Maybe there's closure in that for you. Maybe you'll hire an assassin to kill me for being a traitorous bitch (I wouldn't blame you). But mostly, I'm writing this letter because I need to apologize. For making you feel the way I feel now. For having hurt you more than you were even aware of. I am truly sorry. If you never forgive me, if you go on hating me for the rest of your life, I would deserve it. But I hope that my coming forth with the truth will allow you to make any decision regarding me, or him, with a greater clarity. 
With the best intentions,
P.S. I'm sure you'll never want to so much as see my name again, but I'd like you to know, that my door for communication is open. If you have unanswered questions, I will be as straightforward and honest with you as humanly possible. I owe you that much, and more.
[Edit: Kate was aware only that I was his friend. That we had slept together once during a time they were broken up.]