Dear …
I really feel terrible about my behavior on __(date) __ and I have to send you a personal note of apology. I had no right to blow up like that. It was uncalled-for, it was inappropriate and hurtful and there is no excuse for what I did.
I am sorry for acting in such a foolish manner. I made a huge mistake and I am sincerely sorry for it. My only hope is that you can find it in your heart to forgive me.
Sincerely yours,