Originally Posted by kitten_Val
First, having the stock trailer does NOT mean she doesn't feel claustrophobic. :)
I can't give any advice because NOTHING worked for my paint either (I tried bridge, tarp over, etc. etc. etc. ) for several months + 4 trainers (2 are specific for loading issues and they could NOTHING with her). Only the last trainer was able to make her somewhat over the fear. He made her working around the trailer so much that she'd consider the trailer as the only place to escape the work, so she eventually loaded.
Boy, sounds like you've had some bad luck with trainers! I suspect the last trainer did little if anything to get her over her fear, but just made it the lesser of the evils? This method works for many horses, if it's very consistent, as you saw, but it doesn't create a confident & willing horse.
My method(which sometimes takes a while if they've had previous bad experiences, but I'm yet to fail with - I'm a trainer, so have had lots of experience) is basically reinforcing whatever the horse will give and then asking it to back off(approach & retreat). Do this over until the horse is willing & confident to do that on cue, before asking for a little more & repeating the process.
If the horse backs off without being asked, I do keep a little pressure on the lead to make it uncomfortable, but I don't try to prevent it going backwards - this is just fear talking & I don't think it ever pays to(try to) force a scared horse. Once the horse has finished backing, I will rest a couple of seconds before asking it forward again, to just shy of the point we were up to & work from there again, perhaps more gradually this time.
Keeping sessions short & easy, not getting too hung up on the trailer - using it as just one of many obstacles also helps.