I'd have thought that saddleseat would be a lot more taxing & uncomfortable for someone with knee problems and short stubby legs but maybe I'm wrong - honestly not something I have any real experience of other than 'giving it a go' a few years back.
Its not something that we do in the UK - just a couple of places that do it and they have to travel abroad to compete
Western might suit you better but unless you want to compete you can trail ride (as we do) in english saddles and if you want to challenge yourself more (I would seriously die of boredom just going round and round an arena) then try anebels
suggestion and look into dressage or on a completely different route - why not horse versatility competitons?
Anything will feel uncomfortable to start with - you have to build up and train all your muscles and ligaments to adapt to it.