A little too late, but it was very irresponsible for your parents to just plop you on any horse without ensuring you had proper training.
What you need now is lessons, and lots of them. Either with a trainer going to you and your horse, or you going to the trainer and learning on his or her horses. And gaining experience will not happen overnight.
In fact, the best thing to do would be to sell that mare and keep on with lessons. Once you are no longer a beginner and know what you are doing, purchase a horse that meets your skill level. And when you purchase that horse, bring along the person who has given you your lessons to make sure the horse is suitable for you and sound.
That is my advise. Take it or leave it. Knowing most people, they ignore advise that isn't rainbows, butterflies, and happiness. Too bad, since reality is never so fluffy and sweet.
You are in a dangerous situation, and you don't have the experience to understand that. Sure, owning a horse is cool, and probably every kid's dream. But it isn't a walk in the park. Putting a beginner on a horse not meant for a beginner is practically begging for a serious injury.
I am guessing your parents don't know anything about horses, other than the fact they look pretty and go neigh?