The changes in diet may be helpful as well as getting a trainer to work with you both.
However, in the end it will still be you having to deal with her. If you don't feel like you can be the boss mare, you should sell her. Otherwise she will get more and more difficult to handle. You want to be able to move forward in your horsey education and have fun. Biting, bucking and rearing are so, so not fun.
I think you mentioned she was a rescue - will the rescue take her back to be re-homed?
And think older horse for the next one. My first horse was 17 and she's worked out great.