Ok, well first you need to figure out what the difference is between your tongue and your fingers. It could be that she prefers something...harder and more solid inside of her. Also, ask her what you can do better, what the difference are for her.
The best was to improve oral, aim your tongue up when you do it, the g-spot is about an inch or two away from the entrance, it's easier to reach orgasm when that spot is stimulated. Also, there is the clitoris, it is avoid the entrance hole, above where the pee comes out. You should be able to see a fold of skin there, the nerves are actually very deep inside, so you have to stimulate it thoroughly. It is possible to reach an orgasm through this, so don't underestimate it.
Maybe rub her clitoris while using your tongue on her.
Here and there