I definitely agree with Speed Racer, first talk to someone who know you and knows horses. Maybe you could find another instructor or have an experienced friend?
Don't worry, I really know exactly how you feel! I had a very similar problem when I was your age. I had a lovely pony who I adored and never had a problem with, his only issue is that he was afraid to hack without other ponies, and while we were out one day the person I was riding with decided to canter off without warning me, I tried to hold him back but he was scared to be on his own, and bolted to catch up, which I didnt blame him for at all! But I fell off and injured myself, and because of my injuries I wasn't able to ride for eight weeks. But the time I was back in the saddle I was a nervous wreck, no matter how well my pony behaved....and when he didn't behave it just got worse.
Do you know anyone who has a very quiet and reliable horse or pony that you would be able to take a lesson or two on? Shortly after my fall I got a rescue mare as a field companion. It turned out she was very steady to ride and completely bombproof and my instructor said that she saw an improvement in my confidence on my gelding after i'd been riding her. She really made the biggest difference, and she is my angel
If there are certain situations where your gelding is likely to be more over-excited then try to avoid them for now (riding in large open areas, near other horses, etc). If you can find an instructor ask if they can run you through some confidence building excercises. Also, if I were you i'd make sure your groundwork is tip top, the better your relationship on the ground, the more trust you can have in the saddle.
I don't know what country you are in, but you could also look into finding a product called 'Bach's Rescue Remedy'. It's like an oil that you can put a couple of drops of onto your tounge and it's supposed to have a calming effect and help you center your energy. You can get it at pharmacies around here, and you can also use it for your horse if necessary.
I liked to keep a good luck charm on me as well. I had a necklace with a horseshoe pendant, and knowing I had it with me/giving it a little touch every now and again made me feel better. I have a friend who believes in crystal healing, and she was always kept a very small piece of rose quartz in her pocket, as it is supposed to have emotional healing properties.
I don't know if any of this will help you as everyone is different, but the best advice I can give you is DON'T GIVE UP. If you really love your pony and you feel like you can keep hold of your relationship you will have to keep at it, because it will be difficult at times.
I had a rollercoaster ride with my boy, but I kept trying and we grew together. Since the accident I've ridden him in a halter, and learnt exactly how to deal with his 'quirks'. I have a lovely mare now and there's no stopping us - it CAN be done!
I wish you the BEST of luck. Feel free to post any other questions and let us know how you get on!