I've so been there! As a kid, I'd run all over the place on the horses without a moment hesistation but for some reason when I started riding again as an adult it got a lot scarier...
I agree with Skip a lunge lesson is a great idea.
One thing I did that really helped me was to have a contingency plan in place before I got in the saddle. As I'd groom my horse I would explain to him what we were going to be doing in our ride (silly I know but it calmed my nerves LOL). I would make sure to tell him, if you buck when we canter I'm going to turn you in a circle or ride it out or whatever I could think of - I never needed to do it, he was a saint. I'd always end my little speech with "and you get lots of carrots if I stay in the saddle"... Al got lots of carrots in my 1 1/2 years learning on him... When I'd get in the arena I would just focus on what we were doing, not what was coming up. I found that the build up to it was a lot scarier than the actual action.
I'm doing the same thing now with my boy. He's fantastic but when I first got him had some bucking and head tossing issues. We've worked through them but what he's done in the past is still stuck in my head. We're comfortable cantering in our little paddock but are working up to me being more relaxed and trusting in the pasture and on the trails...
For what it's worth, I've come off twice since I've started riding again - both times the horse was standing still only moments before I hit the ground...
Good luck - stop worrying!!!