Do I Still Have a Dad?

My dad is (most likely) not breaking up with his horrible girlfriend.  And I am really sad about that.  She treats him like crap.  She curses at him and hits him and forces him to turn against my mom, whom he still cares about.  (They are not getting back together ever [Mom and Dad])  She is an alcoholic and smokes and was caught drunk in public and now has community service.  She is really mean to my dad, and calls him names.
So he's giving her more chances, and I don't want him to do that and I think I'm going to have to say goodbye to my dad.    I don't want that to happen, but it is kind of what I have to do to put myself out of the bad position.  My mom doesn't completely agree with my decision, but she definitely doesn't want me to be around his girlfriend.
Do you think I'm doing the right thing?  Please let me know what you think.