I think that you should work with the same horse untill you get the best possible together. I have worked with my lease horse for three years now and we have come so far and learbed so much of each other. I think you should keep him or if you don't have time to ride him you can use him to riding lessons for smaller kids or just have him as a lease horse for one person so maybe he/she can work with him at his level.
Have you had a physio look at him? I know horses who have done Grand Prix Dressage at 20! Know another who only started elementary dressage at 17yrs. He gets regular physio and backman sessions to keep him supple enough. As a dressage rider I know exactly what's involved and there's nothing very difficult at elementary level. I find back on track and magnetic rugs and boots make a huge difference.
I think if you are wanting to continue growing in your riding it is totally fair to get a horse that can help you do that. Unfortunately, no horse is a perfect fit forever. This is why horses are bought and sold.
If your horse can't physically do it, then I would do what is best for the horse. If you do sell him make sure you check the living conditions of the new home and how the potential new owner rides and handles horses.