This is a pickle. I'd be pretty upset that she didn't mention that it's an awful lot of money for a $15 fill. I mean, great tips are wonderful, but a $20 "tip," which is greater than the actual service, should have caused question. On a high end, I might just pay an even $20, which is still a pretty substantial tip and greater than 20% and leave it at that. $18 is perfectly fine. I would be loathe to drop her at this point, given the nail troubles you have and her ability to find perfection, so she gets a pass. I don't know that I would really say anything unless she mentions something or has a look, in which case you can explain you didn't realize all this time you were paying for a full set, not for a fill. If she does notice the drop, you will also be aware she noticed you were paying double and never said anything, and you can make your choices from there.
If she truly never really noticed you were paying double+tip, she also won't notice you are now paying the regular fee+tip.