My mother was exactly the same way. She'd bring a dog home, get mad at it for not training itself to be perfect, and then get rid of it because it was "stupid" or "trouble". She's still doing it to this day! She thinks I got lucky when I got Jake and Max because they're so good - she doesn't realize that it takes a good owner to make a great dog!

If your mom's anything like mine, then I think you should just give up on the idea of having dogs while you're living at home. Some people just don't understand the kind of relationship that people need to develop with their dogs if they want them to be good pets, and as Aly said they think they're just "objects that you can throw away when you're tired of them".