My 1 yr old pit bull won't listen to me, she tries to fight me, teeth barred, . I am at my wits end. help?

I have tried to be like Cesar Millan. But I can't keep up with her. She gets two walks a day... but sometimes she'll still go crazy. I know it's because of me, most of the time this happens i'm stressed. But I can't stop her. I really don't know what to do.