If sh's trying to barrel to the left, use that left leg and tap her her so she quits moving to the left. If it's really bad, have a crop and touch it physically on her shoulder (or your toe if you are feeling secure in your seat) to get her to quit 'dropping the shoulder'
I recently visited my horse and he was doing this.. had a few good whomps to quit it and after that he was fine.
I really think you should save for riding lessons though. Bucking can get out of hand really fast if you aren't sure why it's happening or HOW to fix it.