Show me someone without some regrets and I will show you a liar. What you have done you have done, you cannot change it, you may, in some cases be able to make reparations, you should be able to learn lessons from them, but what is done is done.
maddyone if you went back and tried to learn from your mistakes, it would only work for a week, max a month. Your new decisions would lead to other events and situations which you didn't face first time round and so you wouldn't have learnt anything, you would just live an entirely different life and make different mistakes.