As important as it is to develop a natural ability to memorize, it can’t be a substitute for reading. As you found, it leads to errors, and the intellectual aspects of the music aren’t learned. Miss Lawrence ran into this frequently, and her approach to handling it was what she wrote in her ABC of Harp Playing. Have the student point to the note and say it as they play it. You have to work at their reading skills, and sight reading too. It sounds like you have tried a similar approach. Perhaps giving her a piece and not letting her take it home, only showing it in the lesson might work. You can also try covering up all but the part about to be played with another piece of paper. I think you will conquer this in time. Having mistakes pointed out tends to cure, as no one wants to be embarassed.