Need help with in ex boyfriend?

So my boyfriend (ex) and I recently broke up. (He broke up with me, because of all the fighting) I loved him so much. And I still do. But he's moved on. He told me he really liked this other girl, that he was actually talking to (as a friend) while we were together. She told him she loved him as he was the greatest guy she's ever met. I'm pretty sure they're gonna start dating. But anyway, I wanna write him am apology letter. It needs to be long. I truly am sorry for everything I've ever done to hurt him. I'm not trying to get him to wanna date me again, because I want him to be happy. And I know I should probably write a letter myself, to show how I FEEL. But if someone writes one and it's not really how I want it to go, I'll change it up a bit. Please, and Thank you.