Try to act as a mediator between them, maybe.
I hope it doesn't feel like I'm trying to shoot you down, Unknown Target, but I would actually recommend against this one, to tell the truth. One of the major focuses of my research has been the relationship between parents and their children as they grow into adulthood. One major finding has been that even fully-trained, competent professionals are not good candidates for being the mediators for their own parents. They are too close, there is too much emotional background, and the dynamics are too complicated. It actually usually makes things worse in many ways, and the person it takes the worst toll on is the child who has tried to act as the mediator.
Stealth, I am so sorry to hear that this has happened. Whether or not you blame yourself, it is a frightening experience and tears at the fabric of what has been your world for so many years. You are not abnormal, crazy, or weak for experiencing such strong emotions in response to this. It is a normal reaction to having your primary attachment figures on the verge of a breakdown.
As for what to do about it, I think you have already started doing some useful things. First, talk about it with people you trust. All they have to do is listen, but don't bottle it up inside or bury it. I also recommend writing down your thoughts.
I like the suggestions people have given about not attacking your mom or your dad. I think that is spot on. At the same time, it may not hurt to write them both a letter stating what it was like for you to see that and what your thoughts and feelings about what has happened and the possible future are. Express your love, but tell them the truth about how it has affected you. The purpose is not a guilt trip, the purpose is to maintain your relationship with each of them.
Part of what they need to remember is that being a parent is a huge responsiblity: gone forever are the days when you can just think about yourself. Whether they get back togeter or break up, they need to remember that HOW they do it is going to be very important and have a big impact on the kids, no matter what their age is. So, getting a loving, non-condeming letter from you in which you express your love and support for each of them and in which you have the courage to let them know how this has affected you may be a good way to go.
As others have said, ultimately this is something your parents are going to have to figure out. Don't hide from your feelings, but don't live in them, either. Give yourself time to think about, talk about, and write about what is going on, but also make sure to take some time doing things that help you feel better: I'm not talking about getting drunk. I'm talking about watching sunsets, playing soccer, playing FS, talking to friends, making cookies, or whatever. You need to make room for this situation in your life, while not letting it become your life. Does that make sense?
I sincerely wish you the best of luck. One of the tricks to getting through this sort of thing is not trying to run away or numb out. That which we flee from will always have power over us. That which we are willing to face and experience is that which we will eventually overcome.
