Dear {Recipient},
I won't waste your time and I'll just cut to the chaseI'm sorry I upped and left.
You probably won't believe me when I say that I've felt shitty ever since that day. But even though I made no attempts to contact you for {amount of time}, I've thought about you every day and wished that I've done things differently.
I know that what I did hurt you really bad, so I'm not expecting a reply. I understand. I just wanted you to know that I'm really, really sorry.
{Your Name}