Practice the gallop during your hack and work your way up to galloping over poles on the ground and work your way up to jumps!
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.