maybe then if you agist somewhere see if anyone you could get another horse put in the paddock while you ride like a companion pony or retired horse that might be sitting around or maybe even someone's horse that is quite brought to keep yours company?
Buy another horse? Hate to say but that's the fastest option, breaks up the buddy buddyness. Also maybe have your other horse wiring sight of the one you're riding so they can stick their headband over the fence to see. Maybe also give them hay in a haynet or a feed to distract them