If I could tell him anything...

I would say:

You know you want to...it's all you can think about; that you made a mistake.
A BIG MISTAKE.
You're sorry, maybe not for walking away because that's what you "had to do", but for hurting me and all you want to do is let it be known. So stop letting your ego get the best of you. Put your foolish, childish pride aside and make well with the world. C'mon.
You know you want to...It's all you can think about.