Say sorry to your friend by sending this sincere sorry letter.
I know that there’s nothing I can say that can make things better. Still I must apologize, not because this is the right thing to do, but because I am sincerely sorry for what happened. I have seriously lost so much sleep over this, it kills me that I hurt your feelings and I've put you at such an awkward spot. I've messed up big time. I wish I could undo what I've done. Please understand that our friendship means a lot to me and I would do everything I can to set things right again between us.