He is NOT in constant pain, and not always in pain when riding. What causes the pain is asking him to do a lot of work collected and to have impulsion from the hind. He is to be ridden, preferably at a walk with only short bursts of trot and canter holding himself however he is most comfortable and not "pushing" him. Basically be a trail/play horse. The more he is exercised the less pain he will be in, as long as I follow the rules of not asking to collect and use his hind end so much. It's kind of opposite everything we have been doing, but it's what he has done for himself all along.
I am asking this not to be mean, but to understand, and out of concern for Cinny.
How does anyone really know what sort of pain he has and when, the vet can give an opinion, but Cinny is the only one who really knows.
AS to working in collection or frame, I'm a little confused, everything I have been taught tells me that by trying to get some collection and a more rounded frame a horse can carry a rider a lot easier, certainly easier than being strung out, having a head up and tightness in the back.
As hard as it is when you have fought so hard and so long, are you the rider that Cinny needs? Watching the last video posted it looks like many that you have posted before, you just don't gel as a team, your needs and styles seem to be way off, doesn't make him a bad horse or you a bad rider, just not great together.
You were on the point of selling him before the Lymes diagnoses, maybe you should revisit that point, and ask what is best for both of you at this point, maybe he needs someone else to take him along the next part of his journey? You were all for it then, finding yourself a partner who can take you along the dressage route, and a new role/home for Cinny were he can be happy as well.
Just thinking out loud and offering an outsiders point of view.