Were you on any substance when you left? That can cloud our judgement.
You ask how you can get over this mistake. I think only time itself will heal these wounds. We often have to pay dearly for what we do; it's just the way it works. Try to think of it as a learning experience and that you are now older and wiser and won't make the same mistake again. Until you get over it I guess you'll just have to keep thinking about it.
Try and be thankful for the positive things, daily.