I want to stress this point: I have never tried what I am about to tell you, but based on training books and training classes, this seems to be a logical step (but maybe it's not).
During those times that you are going to take her out, have her walk to the door with you, ask her if she wants to go "outside", have her sit by the door and give a treat at the door, before you open it. After you give her a treat, tell her she is a good girl, open the door and take her out to go potty (give her a treat to go potty or whatever you normally do).
I would do this every single time I take her out. Give her a treat for walking up to the door after you ask her if she wants to go "outside" and another one for going potty outside. She will at least begin to associate going to the door with treats and begin doing this for you. But hopefully at some point she will associate going to the door with going outside and outside is where she does her business.
The problem you may have is that she will go to the door even if she does not have to go to the bathroom, because she will want her treat. But I think that is still ok. You give her her treat, open the door and let her out for 5 minutes and bring her back in. (you will have to kind of taper off on this, because you do want her to associate going to the door with going outside potty).
After awhile you should be able to tell her she is a "good girl" instead of the treat.
But if it's any consolation, Millie knows to go to the door to go outside and potty, because she did it yesterday, but this a.m. she had a pee accident in the living room, about 8 feet to the side of the door.
