How we cite our quotes:
[Edward to Bella:] "But…" His fingers touched my lips lightly, making me shiver again. "There are other hungers. Hungers I don't even understand, that are foreign to me. […] I'm not used to feeling so human. Is it always like this?" (13.172, 174)
This appears to be the first time Edward has been sexually attracted to someone. Since he's never been in a relationship before (and he's over 100 years old), he doesn't know how to go about dealing with these new emotions. He's not even sure if his feelings are normal.
Edward: "I have human instincts – they may be buried deep, but they're there." (13.182)
Edward isn't used to having to deal with human sexual feelings. And since he's been alone for so long, the feelings are strange to him.
[Edward:] "No," he continued, " I was thinking there was something I wanted to try." And he took my face in his hands again.
[Bella:] I couldn't breathe.
He hesitated-not in the normal way, the human way.
Not the way a man might hesitate before he kissed a woman, to gauge her reaction, to see how he could be received. Perhaps he would hesitate to prolong the moment, that ideal moment of anticipation, sometimes better than the kiss itself.
Edward hesitated to test himself, to see if this was safe, to make sure he was still in control of his need.
And then his cold, marble lips pressed very softly against mine.
What neither of us was prepared for was my response.
Blood boiled under my skin, burned in my lips. My breath came in a wild gasp. My fingers knotted in his hair, clutching him to me. My lips parted as I breathed in his heady scent. (13.231)
Edward and Bella are testing their limits. Edward constantly worries that he'll accidentally hurt Bella if he gets too close to her. But instead, Bella seems to be the one that's unable to control herself here.