I am getting the picture of "operator error" here. You mentioned that the BO is very knowledgeable. Has he seen you try to bridle your mare?
As I posted earlier, my similar problem was created by me. Trying to "pull" the bit into the mouth too fast, banging the teeth, forcing things. I had the one episode when I could not get the bridle on. I honestly spent 3 hours trying, with my mare putting her head up out of reach, pawing and backing up to get away. I had no clue what to do but had in my head I HAD to get that bridle on. I ended up having to be satisfied when my mare stopped putting her head out of reach when I held the bridle, up to her face, but I stopped trying to get her to take the bit, only stand with me holding the bridle in position.
I did not sleep that night at all. I was sure I had ruined my relationship with my mare and I would NEVER get the bridle on again. I had a lesson scheduled for the next day. I can't tell you how upset I was because I KNEW it was I who had done something wrong and thought my mare was going to get worse and worse.
The next day, my trainer, who is short like me (5' tall), calmly took off my mare's halter, but buckled it around her neck to use as a hand hold. She held up the bridle. My mare objected. My trainer used the halter to pull her in a circle around her. Then tried again.
This went on for about 15 minutes, then my mare accepted the bit. We let her rest for about 5 minutes. Then the bridle was removed, we rested, then my trainer repeated the bridling. This time it took only 3 attempts and circles before my mare accepted the bridle. We rested, then removed the bridle and tried again. This time, only one circle.
Then it was my turn. My mare tried to evade, but I used the circle method, only one turn around. I was able to bridle. The next attempt, no circle necessary and we quit for the day, no ride. The next day I was able to bridle without my trainer present.
I also realized that I had been letting the bit drop when unbridling and it was banging her teeth at that point as well. Now I hold the bridle after it comes back over her ears and wait for her to open her mouth so I can guide the bit out and not have it touch her teeth.
I am not a beginner. I have had 22 horses in my lifetime and never ever had this problem before. I had been away from horses for 20 years until 3 years ago and have been surprised at how much I forgot, how much I never knew and how lucky I have been.
I know there is more than one way to solve this problem and this is how I solved it for me. This happened 3 months ago, and every once in a while my mare will still test me on the bridle. Just one circle is enough of a reminder.
I am wondering if putting your arm over her neck is an issue. I am too short to do it that way (yeah, I know, the head should be lowered) but I wonder if your mare feels trapped or there is too much downward pressure that is not getting relieved and she is objecting to that as well?
Someone needs to be watching you and helping you. There is no substitute for hands on help. Good Luck!
P.S. I just read what you posted. I think someone other than your trainer needs to watch you and you need some other ideas because backing her up isn't working.
Last edited by Whinnie; 10-01-2015 at 10:33 AM.
Reason: add information