It's hard to accept this, but you really can't change the past. 
You can't go back and manipulate things to the way you wanted them to happen. 
Because life'd be meaningless and boring and just not worth living.. 
But you can change the future and that's a beautiful thing about life. 
Yes, you will make mistakes ! And yes, you will have really bad days ! 
But as long as you let the past go, you'll have such a gorgeous and bright future ahead of you.. 
Forget the past, remember the lesson and most importantly keep moving forward...