Dear {Offended Party}
I’m sorry.
It was wrong of me to {state offense} to you. You did not deserve it, and you certainly did not deserve it from someone who calls you their {offended party’s relation to you}.
I can’t say what made me {say/do} what I did. Whether it was frustration, momentary anger or just a lapse of better judgment, my actions were unnecessary and infantile.
If you can bring yourself to forgive me, I can promise that something like this will never happen again.
Your {your relation to offended party},
{Offending Party Name}
