I understand.....years ago when we got a doggie door, we worked and worked with our pug, and he couldn't seem to get it. Our cat walked into the room, saw what we were doing, and pranced right out without a thought!!
I found having one person on one side of the door with the dog and another person on the other side with a treat that she can't resist. Show her the treat and the only way to it is through the door.
Do that a few times, and she'll know she can go through it, and hopefully she will go through on her own when she needs to!