No control over my horse when jumping!?

I am getting a little sick of some of the stunts my horse pulls. He is fantastic with the flatwork, but as soon as we start jumping he completely loses his mind and runs away as if something is charging after him. I've got zero control as he runs flat out. I turn him in smaller and smaller circles until he stops. It isn't a training issue - we've been jumping for a while, though not higher than 80cm. It is my fault though, because before a jump I get tense and he rushes into it and runs away with me after. When I circle and stay calm, he goes into the jump beautifully, but still runs away with me after, even when we do little jumps. I have also tried turning him into the fence, halting and then reining back, but as soon as we try jumping again, he loses the plot again. I am terrified of him at the moment, even though he is the sweetest thing doing flatwork. I am jumping him in a Waterford bit. My instructor got on once, but he doesn't do anything remotely as silly, so it must be me. I try to stay calm, but in the end I feel like bursting into tears. Please, no stupid or rude answers like stop jumping. I do enjoy it when he is sane, (which sometimes happens) but at the moment I am frustrated and scared. Help?