Ex race-horse problems.?

I have an ex racehorse. His ground manner are superb! He's okay when we work in the arena (Sometimes) But when he gets jumpy, or bolty, I do the 1-rein stop and it get's worse. And out on the trail, he gets excited and bucky. I don't know how to help him. He threw me a few weeks ago. I'm a little scared to get back on. How do i fix this problem?