Concealed - you wondered why there was no immediate response. Well that could be because none of us have been faced with your problem.
The first thing you need is an open minded instructor, preferably one who is a physiotherapist.
But let us see if we can make any suggestions.
The rider has to absorb the upward thrust of the horse at the trot. Normally he has three shock absorbers to do that
The ball of the feet and the various toe joints.
The ankle joint ----- which you have't got
The knee joints
and The hip joints.
then, on some western saddles, a sprung seat.
However after your recuperation you may not have the full muscular capability that you might need to make full use of those joints.
You will also need the full cooperation of your horse.
Perhaps instead of trotting you can jog - ie do a very slow trot and by using your toe joints alone you might be able to cope or maybe you could do a lope instead - ie a very slow canter
But, Oh My, to be able to tell you what to do over the internet, that's a big question. You have an individual problem and what is more so does your horse because at first it might not understand why you are riding in the style you do. As you say - it has been off work for three years so maybe it isn't that keen to get back into work. Maybe to get you competent in the saddle again you will need a brain dead school horse - something mature, unflabbable, rythmic and reliable.
By my book, you are doing well to mount up.
Many of us ride with toes down and we carry the weight in the balls of our feet.
Some of us have very little flexibility in the calves or the hamstrings. We ourselves don't have the ideal flexibility to use the ankles as we might choose. But we get by.
Here in the UK we have some riding instructors who specialise in training handicapped riders to ride and amazingly those riders come to give a pretty good show in the dressage arena. But those instructors have special expertise.
What is more the horses are also very carefully selected for the roll. I have personally watched a rider with no feeling from the upper thighs downwards do a perfectly acceptable dressage test. But I cannot tell you how he did it. Before the demo he had to be lifted on the saddle and afterwards he had to be lifted off. What is more it would have been very rude of me to ask.
If you are keen to pursue your hobby, then you are going to have to become an expert in your own condition. If I were in your position I'd be looking for a physio who rode horses. Then maybe one day it will be for you to tell us about the solutions you discovered.