The radio station I listen to in the mornings (and pretty much always, but this involves the morning show) was on the subject of regrets this morning. Did you know that the average person spends almost an hour a week thinking about their regrets? That's 52 hours a year- over two full days out of every single year.
We all have things we wish we hadn't done. Or that we wish we had done- or handled differently. The more time we waste on our regrets, the more time we lose of our lives.
Can you do anything about it now? If yes- then do it. If not, move on! I realize it doesn't seem that easy, but what is the other option? Spending two days a year for the rest of your life worrying over things you can't or won't change now?
Everything you have experienced, both good and bad, make you who you are and have brought you to this point in your life. If you are having a rough time, you are learning, you are toughening up, you are forging a new part of yourself.
Don't waste so much of your life thinking about what could have been, should have been. Appreciate the lessons you've learned the hard way and pick up, move on! There are bigger and better things always around the corner, if we can just see them.