Dear {Recipient}
Words will never fully express how sorry I am, but I truly hope that it is a good start:
I’m sorry.
I regret nothing more in life than what I have done to you. It was wrong, stupid and immature and you do not deserve any of the grief and anger I have caused you.
I can’t bear to see you unhappy because of my actions. When our relationship began, I promised {to myself} that I would make you happy. I failed. I failed both of us.
But I want to make amends. I truly do. You’re too important of a person to lose and I hate myself for realizing that now rather than earlier. I understand the gravity of the situation, and I hope that you can find it in your heart to forgive me.
Please call me when you have the chance.
Love always,
{Sender}
{Sender}