Hi there, have you ever tried using a surcingle? I have used these for years. Go back to your smaller round pen, put a surcingle rig on, and start with baby steps. Just W/T at first for a few days. Get your horse used to walking both ways first, then build up to a trot, and stop. Let the experience be enjoyable for both you and the horse. Try it again the next day (same thing) and the next. Even if you rig up the surcingle five times for a 7 minute trot, it will be worth it in the long run. Once your horse gets walking and trotting DOWN, to where it’s easy and natural with no issues, the canter will come. Get your horse used to what’s going on. Personally, I am not a huge lunge line fan, I like to free lunge in a round pen unless I’m traveling with my horse and have no other choice. Think about yourself going into a haunted house at Halloween. It’s scary, and you have no idea what is going to happen while you are in there. That is how your horse feels in the lunge pen if its to the point of rearing, and trying to get out. You want to have a fun merry-go-round, kicks and giggles experience for you and your horse. Attached is a link to a surcingle that SmartPak has https://www.smartpakequine.com/pt/web-training-surcingle-4982
You can also buy a surcingle kit for a couple hundred dollars, but your horse is going to need time in the round pen. The surcingle will really help. Good luck, keep us posted!