Thank you all so much for your input. I have been feeling, privately, very sad about all of this, and the sense of so many understanding people out there willing to advise a stranger is heartening.
I am soon travelling on vacation to Nova Scotia, Canada, where I've located a well-established horse farm offering a 1hr lesson followed by a 1hr guided trail ride. I'm thinking to sign up, and use that experience to gain additional, objective perspective on my abilities (or lack thereof). Does that strike you as a good idea?
I am trying to be satisfied with learning to groom the wonderful horses at my teacher's barn, watching others complete their lessons and drills, and making myself generally useful on the farm in order to spend time with the horses. If indeed I have to face the fact that it's the best I can do, then I will live it - and gratefully, too. I often drive for hours in the countryside near my home just to catch a glimpse of horses in their paddocks. When no one can comfort me, the sight of the grace, majesty, perfection and humility of horses just being horses does.
It's because of my strong affection and affinity for them, though, that I would never want to cause a horse a moment's unnecessary discomfort or confusion by my ineptitude. Golden Horse said in an earlier post once you must be honest with yourself, and this strikes a deep chord with me. I want to, but I simply don't have enough information to do that, as my teacher seems hesitant to explore and evaluate my abilities (or disabilities), but she's too kind to simply send me away, either. This is why I think I'd better try for a separate, objective assessment from those with whom I have no relationship.
I hesitate to mention that, as so many have suggested, it's obvious to me that everything would be easier without the added burden of being significantly overweight. It's true that over the past 3 weeks, while trying to resolve all of this, I've intensified my exercising and dieting efforts, and am now actually 243lbs instead of 250, as of today. It's such a drop in the bucket, and I've traipsed up and down the scale so many times that I don't breathe a sigh of relief until at least 25lbs or so have come off. My policy is normally not to mention anything less than a 10% bodyweight loss, but I don't want you to think I haven't welcomed all of your advice.