What is the best bit for a hard-to-control horse?

I have a hard-headed 6 year old gelding that i have been riding on and off for about a year. He behaves badly and doesent behave well with the bit he has, its just an iron bar with a large curve in the middle.. may be a hackamore but im not sure. Just wondering what type of bit would make him respond better because you have to basically almost tear his head off to get him to stop. Ideas??