Just jumping back in to make a point.
While Cushings is a likely diagnosis, there's nothing "definite" or "1000%" about it. It's the most probable guess, but not the only possibility. There are other medical conditions that cause hirsutism, including in horses, and other possible explanations for loss of topline, distended belly, lack of interest in life, difficulty walking etc. A Cushings diagnosis involves a blood test, not just looking at a photograph and speculating.
It may seem pedantic to some, but it's really not. It's really important not to jump to conclusions, and not to dismiss other possibilities - with this matter, and as a general rule in life.
And as to dietary modifications, that's only necessary if your horse is also insulin resistant/has equine metabolic syndrome, which is not the case for all Cushings horses. I've got a 24-year-old horse who has Cushings and no issues with blood sugar regulation, and he eats as he always has and is doing very well. Cushings in horses is usually caused by a pituitary tumour, and the regulatory problems caused by the tumour depend on where exactly it is, how big it is, and a whole raft of other things.
In humans, cases of Cushings caused by pituitary tumours are usually treated by removing the tumour. Because that's not a realistic option in horses, their Cushings gets treated with a dopamine agonist, which often (but not always) improves the quality of life of the horse significantly. Dopamine is involved in the regulation of physiological processes (in a complex way and not as the only factor) and affects mood, motivation and interest in life.
It's best to catch Cushings early, before a lot of degeneration sets in; but if that wasn't possible treatment can still make a significant difference in many cases.
Good luck,
@Debbie in Lybia - it's so kind of you to give a nice retirement to senior animals in need of TLC. ❤ I've only ever bought one horse in my life - an Arabian who died in 2014, at age 32. All the others have been the debris of the horse racing "industry" which I loathe. That includes my current three, all over 20 and enjoying a stimulating free-range retirement in which they can freely socialise with each other and a small herd of donkeys, and (because we've been lucky) they can explore a whole lot of diverse countryside in the daytime. The socialisation bit is so important - my youngest adoptee spent 17 years basically in solitary confinement, never allowed to interact with another animal in the same enclosure. This changed when we got him. Here's a photo from the morning he first ran with others of his kind, and on actual pasture.
He's in the middle. My Cushings horse is on the left of the photo.