Ask her to simulate how she likes the pressure to be with her tongue on the pad of your thumb or index finger (those are the most sensitive). And then show her what you do with your tongue on the pad of her finger. There's a little bit of a battle going on between perception and reality, what feels light to you isn't light to her and it can be hard to nail down what she's looking for.
Her sensitivity could also be decreased by using vibrators with strong pulses and vibrations. It will take some time, but it does eventually happen. I'm willing to wager that most of us members of the Clit-o-Steel club didn't start out that way (I know I didn't).