Grass sickness? I don't know a lot about it, but it's a very unpleasant disease with an alarmingly low treatment/recovery rate
A lot of ponies and horses seem to be coming down with it this year from what I've heard.
Fluke is environmental, but you should be able to treat quite easily for that. I think a poo sample away for worm/fluke analysis might be needed but your vet will know if it's a problem in your area.
We had an oldie with cushings. Their coat can get thick and fluffy and they exhibit arthritic type symptoms - one tablet a day and he'll hopefully have a decent quality of life for many more years but maybe not for riding. Our old fella was over 30 and on a good day my daughter could get 5-10 minutes on his back while he walked round or a bit of lunging and going over small jumps (not backed) for exercise.
Hope you get some answers soon