I too am in my forties and know exactly what you are talking about. I have many regrets. Things I wish I would have done differently. Things I wish I could take back. Things I said that I didn't mean. And so on...
As you go through life, you continue on your path. If you spend your life looking behind you, you will continue to fall on your face because you are not looking where you are going.
The best thing is to understand what happened, what your part in it was, and to learn from it. I tell my kids, "A wise man learns from his mistakes, but the wiser man learns from other's mistakes." The best thing you can do is to learn from them and let go. If you need to make amends, you may need to do that as well. Even if the other person does not receive it. It will make you feel better.
One thing I know is that you are alive and breathing. You have a choice today to make a difference. You have a choice to make the rest of your life anything you want it to be. So choose a path that will make you smile. A path that, at the end, you can look back and be proud of yourself. Your journey is not over. Your future is yours, today is all that matters, and the past is behind you.
Yesterday is history, Tomorrow is a mystery, and today is a gift... that's why they call it the present!
Take care!