I am sorry sorry that you are going through such a difficult time. This is unfortunately an adult decision and there is probably little that you can do to help.
However, you can learn to help the situation by taking pressure off of your own parents from your needs and those of any younger siblings. Look for ways to help around the house. Work on your own independence. Cook, do laundry, clean -- learn to bank and how to pay bills and generally all the skills you will need when you are an adult and responsible for yourself.
It is perfectly fair to say to them both that you are unhappy with the situation. It's fine to say you are scared and worried. They are your parents and even though they are having problems -- they still will love you and look after your needs.
I highly recommend not taking sides. Certainly your opinions are yours and you might even be right -- but it won't help if you are applying pressure to your mother. You truthfully only have your father's word about possible affairs, and your mother might possibly be right about him. Or wrong. It is their business. I am not saying extramarital affairs are okay. My first marriage was broken that way -- and all the lies that were involved. I imagine being accused of doing something is almost as damaging as doing it to the relationship.
You must try to let them deal with their relationship and to be the best daughter and person you can be. Use this time to improve yourself and to learn what you want and don't want out of life. It will not be easy. I am not pretending that it will be. You have to grow up a little ahead of schedule.
If you remove your own pressure from the situation and perhaps ask them to get family or marital counselling, then you will have done more than 90% of the children of divorce ever could -- because you know it might happen. Most kids are told when it's already decided.