When my parents decided to separate and then divorce, I was not unhappy with their decision. It's okay to have feelings of anger. Personally, I wanted the divorce to happen and have been much happier because of it. I have been able to have an even better relationship with my parents ever since (the divorce).
Try to remember it isn't your fault, you did nothing wrong. Not every marriage lasts and that's okay.
It's perfectly natural to freak out for a bit. The main thing to remember is that this isn't happening because of you and it isn't really anything you can fix. Your parents still love you as much as ever, they just want to make their lives easier by spending some time apart. The best thing to do, and I learned this the hard way, is to try and stay out of things as much as possible. Don't drag yourself into an argument unless you've got your boxing gloves on and you're ready.