Dear {Recipient},
Let me begin with, “I’m sorry.”
I’m sorry for {state action}. I can see that my behavior and attitude was hurtful and totally unnecessary. You did not deserve to be on the receiving end of my actions.
I am normally not like this. While I am not excusing myself from my {unprofessional/immature} actions, I believe I acted the way I did because of my {fatigue, bad day, etc…}.
I hope we can continue to {work together, maintain our friendship, etc…}. If you have any thoughts on this matter, please feel free to contact me at {phone/email}.
Sincerely,
{Sender}