I'm a little nervous to get back on!?

Okay so my horse has bucked me off four times. He's only six, and my trainer has been working with him. He recently bucked me off and I broke my wrist. He was being boarded at my friends house, and my friend wasnt nice to him. Now he's at my trainers house, and he's calmed down. I'm going out to see him this week and watch her work with him. I get my cast off in two weeks. Of course I am going to get back on. But it's understandable that I'm a little nervous, right? He's not going to go back to my friends house, so that's good. But still, what if he responds differently to me than my trainer? The one thing I have trouble with is being the boss. I don't fully take control, which my trainer says I need to work on. So I've been trying to prepare myself and thinking to myself, I need to be the boss. I need to take control of my horse. If I don't, anything could happen. And, he will be able to sense that I am nervous. So how can I get rid of my fear before I get on him? Do you have any advice for guiding me to a mindset that I am the boss? I don't want this to happen again, because next time it could be a broken back, or broken neck. I don't want this to be a Christopher Reephs story. So... do you have any advice for me? Thank you(: