Also, it really helps if you're riding a horse that can gallop and then easily be brought down to a walk (or trot, depending on what you're comfortable with). Trying to gain confidence on a horse that runs away with you is not a good idea.
I would start small. Maybe in an indoor arena you can canter faster down the long side and shorten on the ends. Then move out doors to a bigger arena and do the same thing. Get comfortable with the faster pace. You can also do jumping exercises where you set up a line a put the distance a little long. Then you have to extend and pick up your pace to the next jump.