We trailer my gelding Bill down for lessons every sunday. He recently was getting a drink from the bucket sitting on top of the part over the trailer tires and hit it and it flipped and spilled all over him, this has happened before but this time, he freaked out. He started pulling back, but was tied so obviously he couldn't do much. I was able to pull the leadrope fast enough that he didn't break his halter or lead. We've started keeping the bucket away but he seemed to have learned he could do this at the trailer. Any excuse he can find, he spooks and pulls back. I've started not tying him and just leaving the lead through the ring. It's getting really frustrating because I can reprimand him and that doesn't work and I've tried using treats and being nice and babying him but he still acts like a snot. He doesn't act this way in cross ties or in a stall, just at the lesson place next to the trailer. I've switched sides of the trailer, I've walked him past the spots he acts like he's afraid of and I leave the water bucket on the ground away from him and offer it periodically.
I'm running out of ideas and I don't want another horse that won't tie so someone has to be with them all the time at shows/lessons/clinics/etc.