Hey there,
I am really sorry that you've been feeling this way. Have you tried to vent your feelings? You could write about them, blog, or express yourself through art or dance. You can also talk to someone about how you feel. This could be a teacher or a counsellor at your school/ University, a social worker, a friend or any adult in your family. You could also call up a hotline. Here is a list of hotlines-
http://www.teenhelp.org/hotlines/
Have you tried to talk to your mother about what has been going on between her and her husband? It is possible that your fears might not be true. But even if they are, you could have your mother's support in dealing with the situation and I am sure she'd appreciate the support that she'll receive from you.
I hope that this helps a bit.
You are always welcome to
PM/
VM me if you need someone to lend you an ear.
Stay strong.