I have a tremendous amount of hostility toward my mother because of my father's abuse?

We really can't know what's going on in anyone else's mind. Yeah, she should have protected you, but did you ever think that maybe she, too, felt trapped and afraid to say anything? It sounds like she's still got a lot of internal problems with this, and that's her burden, but you should definitely seek counseling. Just remember, we can't change the past, we can only learn from it. You'll never be able to convince your mom to "go back" and do things differently. But you can work out your own outlook for the future, and have a much better life without being all clogged up with this poison.