Basically, I have a horse; and I believe it's the sole reason why I'm not able to get anyone to follow me. Every time I try to recruit Lydia or any other follower they claim I already have someone with me, but when I fast travel there's no one else around. (Except my horse.) Is there any way to fix this so I can finally have a lackey carry my luggage for me?

Kill your horse?

I think I tried that before, it wind up coming back to life. (And my horse isn't Shadowmere)