I can't act my part and i have an exam on it!?

In my performing arts class i have a part where my character has a fit after having extasy and dies. I really can't act it. I've been trying for weeks now and i really struggle with it, my group get angry when i want to practice it. I've watched videos and done everything i can trying to do it. But i can't, i can't do it. It would be the same if i collapsed, it wouldn't change the play. But no one is listening. There grades won't be effected just mine. If i did, which i'm not doing it i would have to do it infront of about 50 people and an exam bord. I can't do it! Help.! I'm sat in my room crying because they don't understand. ...