I am your age and a horse I might have described as a fun forward going ride for me 10 years ago I would probably also describe as a dangerous ride for me now, I would also probably flatly refuse to even get on it!
But of course that same horse would still be a great fun ride for someone younger, fitter and more confident than I am these days. I think your physio is really being unfair on the horse but probably because she is concerned for your safety.
Personally I would sell him on to a more appropriate home and find yourself a nice steady well mannered horse to enjoy the short few years riders like you and I have left to realistically still be happy doing what we love. Life is too short to be struggling with issues IMHO.
I'm over 50 and my friend, also an equine physio tried to steer me away from a WB or WB types as she felt an ID type would have suited me better.
I quite understand where she was coming from but it's just not me. I know there is nothing wrong with an ID, they are lovely, but I just prefer the flashier paces of a WB and I used to love people asking me about Bailey and her breeding at shows as she used to stand out in terms of looks, I was very lucky.
I do admit that I would rather have something a little bit on the sharp side (but not too sharp) than something that I was forever having to gee up every five minutes - again not speculating that ID are like that but you get what I mean.
One mans meat is another mans poison and all that....
I wouldn't for one moment recommend anything you had to 'gee up every five minutes' either, for me or anyone else come to that! A steady well mannered horse should be quick of the leg and respond instantly to any aids given, this does not mean a horse is sharp in anyway IMO, simply it is well produced, nicely schooled and mannerly.
Which my ID X is by the way, but then perhaps that can't possibly be the big clumsy old ID sire, but down to his TB dam lol!??
With the extra information, I suspect your equine physio has your best interests at heart. Not that the horse is dangerous per-se, just that the horse isn't a great match for you to ride right now.
I am mid 50s and not as brave as I once was. I went right to an old cob to get going again, now have an IDX. He actually has fab paces, but is surprisingly lovely in temperament for a 4yo, just off the boat. I am enjoying every single ride, even though he is the sort I would have found 'boring' 10 years ago.
So, you gave many choices. You could simply keep the horse as a non ridden one, but you say you would like to ride. You could find a pro, to get him going again. If there are any further difficulties, I would get a vet to do a performance workup.
It is meant to be fun. What we perceive as fun can change over time.
In July just before the very hot spell he bolted when my husband turned him out. Ran across field ,slipped down, caught his face full on to the electric rope fence.we could not even touch him for 2 to 3 weeks. Friend came and Reikied him to which he responded and she got his head collar on for first time. Then another friend and her daughter came. Head collar on and walked him to his stable Been back to 'normal' since that time. I wasn't there when he bolted as still recovering from fall. However, started to go with my husband every day and just sat in field with him. Only thing we can think may have caused him to run off ewas when he went through gate caught his near fore in a bunch of nettles!