I have fibro as well, so I too, have to be careful how long I lunge as well, so I will in corporate sending exercises, and c-driving...(I'm going to assume you are familiar with Clinton Anderson)...that helps me alot. When the horse knows how to lunge well, you can also 'challenge him' further by simply passing the lead behind your back rather than turning with him, and that could help you on some days too. I find the more I can move forward, like with the C-driving, the easier it is to maintain my balance...sending exercises also rarely effect me, but there are days that lunging, or even yielding the hips, or shoulders, I can barely make one pass, and I'm dizzy...I'm so glad my mare is good at it at this point, and she is also very in tune with how I'm feeling as well, so that helps.
To help him get over his respect issue, do alot of backing, shoulder, and hip yielding...I will back my mare down her paddock to get out into the working area...Lol! The other boarders, I think, think I'm nuts...but they also commend me on how well behaved my horse is too...sooooo...it pays off in the end.
Backing, especially, can help a pushy horse learn to stay out of your space...so do alot of the 'hula hoop' exercise, to teach him to back out of your space and stay there until you invite him back in to your space, or until you choose to walk back into his space. With a horse like him, I prefer to walk into him, rather than allow him to walk to me, atleast until I know he understands not to run me over...Lol!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."