I took on a rescue mare with terrible separation anxiety and like the others said it’s just daily practice. You start small and work up. I have just two horses on my property and I’d take my other horse away and we’d come back in 30 seconds, then a minute, then 5 minutes. Never far away. Now I can leave with my other horse and be gone and she is calm. She knows we’re coming back.
THEN, then my rescue girl needed to leave for major dental surgery and that meant leaving my new mustang alone and I figured he’d never been alone a day in his life, right? Once I got the news she had to haul out for this work I knew we needed to start asap for him being alone.
The first time I took her away from him and we disappeared around the barn he lost it! He’d always left her and never displayed any separation issues. Wow was he bothered with her leaving. So, we came right back. Rinse and repeat. Over and over every day for a week. And sure enough in a matter of days he took notice but stayed quiet. Got to the point where I left with her for 15 min around my property and he was fine.
I will add I do turn them out on grass so they are more distracted. The rest of the story is cute.
The day of the big test was me hauling her out and him watching us out the driveway the whole way. He stayed calm but was out grazing (muzzled). I dropped her off and came right back home to be with him while she was gone - 5 hrs or so. I did all kinds of barn work and he kept an eye on me and I kept assuring him she was coming back.
When it came time to haul out and go get her I pulled his muzzle for EXTRA special focus on “Oh wow I get to eat this grass!” When we came back he was happily out grazing all alone.
It can all be done. Keep it fun, positive, and it will take time.