as sourpuss alluded to, just keep at it. Truth is, you might be big, you might be strong, but you're just not very good yet. But don't worry. nobody ever is. You'll get better every time out. Trust us.

and ps - the goal isn't to WIN or establish your dominance or hurt a guy in sparring. The goal is to help each other get better. ie, work TOGETHER in a trusting way. And frankly, to have fun with it.