Dear XXX
I owe you a sincere apology for my XXXX mistake.
It was never my intention to create such an awkward and uncomfortable
situation. I hope that I can be of assistance in correcting my mistake
and resolving the matter.
While I have caused this unpleasant experience, I am able to see where I went wrong. {State the cause of the mistake if possible}. I believe acquiring this insight will allow me to learn and grow and prevent similar issues in the future.
Please feel free to let me of your thoughts and concerns over this matter. You can reach me at 13x xxxx xxxx.
Yours Sincerely,