If you are fighting to get the bit in the pony's mouth, you are creating rather than solving a problem. Think what you would do if someone was trying to force something into your mouth. Using these tactics, the best you can accomplish is a submission resulting from giving up.
The first thing I would recommend is to get a veterinarian to check the horse's teeth, pole, ears, and anything related which may be adding to this issue.
Once health problems are eliminated, I would start work on getting the horse to relax. Begin by gently -- but not too gently, which may feel like tickling -- rubbing your pony's body in small circles. Begin near the withers and work your way up the neck. It she gets resistant, retreat. Begin the process again. Take as long as it takes. If you rush, it will probably take longer. Once your pony allows you to gently rub her ears, work similarly with her mouth. In doing these things, you will be building a trusting relationship with her. The more she trusts you, the easier everything will be.
You might want to read "The Ultimate Horse Behavior and Training Book" by Linda Tellington-Jones. Frederic Pignon, who is noted for his freedom work with several stallions at once in the horse extravaganza Cavalia, tells of his first meeting with her:
"I told her about a problem I had with Fasto who was suffering from stress and she immediately offered to help. I watched her as she entered the stable and not without some trepidation when she said she was going to massage Fasto's ears. I knew he did not like people touching them, especially a stranger, but as she began gently at the base of his ears he lowered his head and a look of bliss came into his eyes."