Sounds like she has control of her own shoulders, which is why it's so easy for her to just... go wherever the hell she wants.
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.
"Strength is the ability to use a muscle without tension"