I admit, I really miss how things used to be. But I can also admit, that I've accepted the fact that things changed.
One lesson we must all learn in our lives is that things can always change. Sure, we will always remember how things used to be, and we will sometimes lose sight of our future because we want to turn around and re-live moments from our past, but there must come a time in which we accept that things have changed. One thing that we must all note in life is that we cannot undo the parts of our lives that have changed, and that may cause us to regret now.
We can't always go back and undo the pain that we have caused others and ourselves, we cannot always mend the hearts that we break back together. What we can do though, is make an effort to live the best life that we possibly can, and to vow to always be better, and to do better than we ever did in the past. Your past is your past, and no matter what you do you cannot change it, but you can create a better future as long as you give effort within every waking moment of your life.