Seriously dude, give her space, if she said no, then you respect.

Slow talk and warm talk with her about this, dont keep this in your heart. Tell her how you feel about what she said, but in the same time, tell her that you love her not for sex. Tell her because you are young and.. because of first time so craving for more once tried.. like some other guyz said, sex like a drug, once you hook on it, you will still want it. ( it's true but also depend on your mind control, i mean self control )

kiss her on forehead, whisper close to her ears, tell her you love her.

hug her, tell her you love her.

once you make her comfortable, sooner or later she gonna feel more open to you again.

GOOD LUCK