When you go down on your girl your full attention is on her pleasure, or at least it should be.
When you're screwing her you're probably mostly experiencing your own pleasure. A really good stud pays attention to his lady, but many of us are self-centered and focus on our own genitals. That's why one of the great advantages of oral sex (provided you're not 69ing) is that you can maintain proper focus.
Proper cunnilingus requires you to have both hands on her body, like her hips, waist, or breasts. You might be inclined to play with yourself while you lick her pussy, but that's taking away from your focus. Besides she might enjoy the extra stimulation from your hands. At most you should limit yourself to the way your hard dick feels as it pushes against the bed.
It's very exciting to explore her cunt with your tongue. You can run it up and down her cunt lips, noticing each detail of this very special part of her body. Then you can stick your tongue deep inside her cunt and get some idea of what it feels like down there. Unless she's in the habit of spreading her pussy apart for you this may be your only chance to explore down there. And by all means pay attention to how she reacts to what you're doing. This is imperative If you want to become good at being her cunt-licker.
Then there's her clit. It's not likely to get erect unless one of you stimulates it, and there's no better way to do that than with your tongue. If you were fucking her you probably wouldn't even notice her clit. Now you can feel the way it grows. And you can feel the way her body reacts when you lick it. How does she like you to do it? With your face against her pussy and your hands on her body you have a much better feeling for her reactions. And you probably are thinking about her pussy rather than your dick.
There's plenty of time to screw later. Right now you should be focusing on her sexual pleasure. Maybe you can make her cum. Maybe you can make her cum three times! She'll be a really nice lay after she's cum a few times and her cunt is nice and wet.