Horse has separation anxiety with another horse whenever it leaves? I don't know what to do.

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?
@maddieomeara sadly I do not have the money to buy another horse...
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
I have two horses that are like this. What I do is I try to separate them for a small amount of time every day.
If you do that every day they will get better.
Maybe try to find something that reminds the horse about the horse they got separated from
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