The flying backwards when you touch the halter is a tying issue. Can she tie solidly and stand, alone, calmy, for a decent amount of time? If she spooks and hits the end of the lead does she pull back, or move forward to release the pressure?
Wild spot, we never had a problem with her until she started using the trailer as shelter.
What do you mean by this? As in she uses the float as a shed of sorts?
Once we actually get her on the trailer and hooked in she stands great.
Okay - Let me clarify. She walks into the trailer fine - Relaxed, happy, etc. It's only keeping her in the trailer long enough to get her tied and the back shut that is the issue?
(Sorry, I keep flicking between float and trailer - Different countries, lol!)
^ If that's correct, then there are a few things you can try.
1. I usually do this anyway. As soon as she flies backwards, put your whole body weight on the rope (I advise wearing gloves!) but move with her - Keep the weight on the rope constant. I also tend to say AH-AH-AH-AH in a stern voice as they fly back. Now - Only release the rope for a moment when she stops backing up - Just a moment so she know stopping was the right thing. After that moment, you need to take her back to exactly where she was before she flew back. Be mean if you have to - Backing up can never give a reward. As soon as she is back in place, give her scratches, kind words, treats, whatever rewards you like to use. Just let her stand and relax - Where you want her to be should always be the most pleasant place to be. If she repeats the performance, then so do you. Stay with her as she flies back, keeping as much weight on the rope as you can manage. Then that tiny release, and straight back in without a rest.
(^ Note - This will only work if she 100% knows that if she gives to pressure it goes away. She has to tie solidly as above before this will work - Or she will just keep running back and fighting the pressure with no clue how to make it stop).
2. I'm gonna keep coming back to the butt-rope, because seriously, it is the most useful tool we have in our loading arsenal. If you are 100% convinced that any type of rope will freak her out beyond reason, then ignore this bit.
Ok. If she freaks out at the butt rope, then I would get a super, super long rope, and some gloves. Stand her far enough back from the float so she is relaxed and just looking in - Not thinking backwards. Bring it around her, gently, so it is sitting in that hollow between her butt and her hocks. Just let it sit there with enough pressure that she can feel it but not enough to be pulling. She has to know it is there - She should not feel it for the first time when she backs up. Also, you need a helper for this - One needs to lead and one needs to hold the rope. I tie one end to the side of the float that the horse is going on, and my helper holds the end of the rope on the other side of the horse, at the shoulder round about - Or at the side of the float when the horse gets in a way.
The rope needs to stay in the same position on her legs as she walks up - Not pulling, but there. If she backs up, the leader does the same as is no. 1. The person holding the rope does a similar thing - They put SOME pressure in the rope, but also move backwards with the horse and let the rope run through the hands (hence gloves). The horse should feel an obstruction, but not enough to cause rope burn or pain. It is just a block. The onyl difference is that the pressure doesn't come off until the horse moves forward again - As soon as she is forward again it returns to 'neutral'.
I had more ideas in mind but they have just left the building. Will try to remember!