I can't do a cartwheel!?

Hi, I'm 13 and I'm a girl. I have a problem! I cant do a cartwheel or roundoff. We r getting tested for these in gym and im just so embared. I really cant at all i fall on my , my best friend laughed at me, and i was really burt, i dont need my whole gym cl laughing. Im the only one who cant do one. And also im scared and can do it because im so afraid i stand their for like 5 minutes then can do it, which is also embaring. We are doing them in vym cl. Someone pleaseee hlp? Ive practiced so much and i still cant do one. Its so spastic. HELP!!!