It can be a frustrating process, and some dogs learn much faster than others. When I got my first GSD many years ago I was living in a flat (Mad I know) but she was brilliant. I now have a house and big garden but my little westie has taken much longer to get the idea. She is fine as long as I leave the back door open but its freezing.
You really do need patients with this and you need to watch her every second that you can.