(1)  You are doing the right thing and building the right muscles.

(2) Solo had the exact same struggle, it takes a long time to build that strength.

(3) Variety is cruicial.  He may be a bit burned out and sick of it, so make sure he gets trail days and time doing other things.

 Something that has been a great tool for me is to set up a video camera on a tripod when I'm schooling certain things.  If you don't have one, perfect time to ask for one for Xmas!  I can see what is going on (which is often totally different than it feels, LOL) and see what I am doing right and wrong. 

I often struggle with the same fear, of screwing up my horse who is always trying.  But give yourself a little slack -- as my jumping trainer recently told me (who has trained a LOT of horses), the people who worry about messing them up, rarely do.  Horses are forgiving as we muddle through our issues.  But training is NEVER a linear process; it gets better, then worse, then horrible, then better, then great, then worse again, over and over and over.  So don't take the bad days too hard, just keep working at it.

I absolutely agree that the training process with get better, than worse, then somewhere in between... it will always be a work in progress so don't beat yourself up. The fact that you care so much and are putting in so much effort means a lot. The video taping is also a great idea, and has helped me alot. Especially when you can't spend a lot on trainers riding your horse and lessons. I always make detailed notes after my lesson so that I can vizualize everything and refer back to how it felt at that moment.

Good luck :)

 You've got some good suggestions. I especially agree with the variety. Change up the exercises - and even the order in which you do them.

 Some additional exercises (to help strengthen either the back end or lift the back - those are what you really need): hill work, leg yield - either sprial in/leg yield out on a circle or from 1/4 line to rail, trot lengthenings across the diagonal (and back to working on the short side), and at the walk 1/4 turns on the hainches (I do a square w/ these turns at each corner). Don't try to do any really collected work until the strength has built up - just work on staying off the forehand with the back lifted.

One other thing you might try at a lesson is to ask your instructor to watch what you are doing as much as or even more than the horse. Many instructors/trainers are really only concerned about the horse (as long as you don't have outrageously bad posture or position). If you instructor could watch for any stiffness in you various joints (tight elbows is a problem of mine, but I know people who have issues w/ tightening a knee, hip, shoulder, or not using one side enough), especially during transitions.

 I agree w/ 48 - you are way ahead of the game. Transitions are the hardest part. Maintaining the gait just takes a build up of strength and endurance (for you and the horse, lol). A nice upward transition, especially if it can be followed by an active downward transition, is huge.

Yay for you!

(And if your chiro has any suggestions for stretching exercises for your specific issues, doing them just before riding will make things easier for you and your horse.)

 I think that firmly insisting that he keep his attention on whoever is handling/riding him will get him back to being a good riding horse.

Don't sweat the small stuff!   You are right to sometime stop and take a breather.   Horses are like kids, they learn in spurts.   You have to review, review and review somemore.   When you're starting each session, keep it interesting, but allow the horse to stretch, bend flex in a longer lower frame.  Then just as any athlete when they are warmed up and ready to proceed with learning you can move forward into a more 'disciplined' attitude.    For me, my horses and I are spending a lot of time in big fields... we do long trotting just to leg up and for me to balance, without affecting the horse.   Getting out of the arena is to me the BEST attitude improver there is.   When I want to school, I'll find a little level spot, do some circles, serpentines, canter departs with a few strides of canter and lots and lots of transitions.    Then we go back to our long trotting, and perhaps go check on the cows or something fun.   

Give him a reason for doing something...   Cantering is part of what a horse can do....  cantering in balance with the rider takes months (sometimes years) for a horse to do.  Gentle slopes allow you to ask for a canter going up and the ability to help the horse 'push'...  just a few strides is all that you want to ask, then return to a walk.   Hill work also aids in building muscle. Circles round trees rocks bushes...   gives him a reason for staying 'out' on a circle, and you can work in and out in diameter of circles...    logs are a great aid in getting a horse to 'jump into a lead'  Allowing a rider to straighten the horse, change the aid and get lead changes...(simple little logs or  big poles of 6" diameter or 12" diam.)   But, consistency of the rider - are you balanced, are you centered and making it easy for him...   or do you struggle with position, stability and are you consistent with your cues.   You're both learning something "new"...   so, relax,  evaluate how you are riding and go back to basics, establish a firm plan for every cue - Think about how you communicate with your horse.   Exagerate your cues to begin with, and as he learns to respond and you praise him loudly, then as he becomes more and more consistent you can refine the cues to simple weight shifts or movement of leg and seat to ask for the transition.   It's what great dressage or reining riders make look so simple when you watch them.   Minimal effort but their control of the horse's actions is amazing, it is their ability to control their own actions which allows the horse to move so well.   

You're on the right road, wanting to imprrove.   Useful tools are video - have a friend video your rides, then you can pick yourself apart - see what's not going well and what actually isn't so bad.  Having an instructor who will work with you to attain your goals is also invaluable.   Remember to keep it FUN for both you and your horse.