I’ve seen how miserable they can get, and I feel your pain. My guy gets worse as he gets older, and he was so fixated on a mare that I recently had to move barns. He lost a significant amount of weight despite our best efforts, got beat up by other horses, and just generally made himself miserable. He’s never studdish, he was gelded 20+ years ago, and he’s quiet as a mouse and happy as a clam when turned out with the mare, but he worries and frets over every little change. He’s made himself lame pacing fences, too. He can get a little attached to certain geldings, but it’s far worse with mares.
I’ve had to do two on-farm separations from a mare, and both were miserable for all involved. The first time, it just took time. Despite our best planning and management, he fretted, lost weight, and made himself lame, as mentioned above. It took a few weeks before he was himself again, and that was even in a group of new and quiet herdmates. The second time, he lost even more weight, almost two numbers on the Henneke scale. He was less frantic but more visibly depressed. After a few days of calling, he spent his days not moving from one corner of a very large field. I tried to be patient, but I was more worried the second time because he’s in his 20s now, and I gave up and moved farms. We tested for Cushings, scoped for ulcers- every test came back negative, so it appears that it was all stress. A friend affectionately calls it Mare-Induced Senior Gelding Psychosis. He gained all the weight back within a month or two at the new farm, on the exact same amount of feed. Interestingly, it’s not all mares. He’s currently stalled next to a new mare and they can touch noses through their stall bars, but he’s not stressed at all when they’re separated.
As far as solutions, I think it’s an individual thing. It took creativity and patience, and he was getting excellent care and plenty of turnout at all farms where he was boarded. I found that for my guy, he gets most attached to the horses who aren’t taken out and ridden frequently. I also have some success stalling him next to horses who aren’t his pasturemates. When he’s out 24/7 in milder weather, he seems to get more attached than he is when they’re stalled at night in colder weather. It’s frustrating, and it helps a lot to have an understanding barn manager who is willing to try different things and keep ahead of the plan when things change.
Best of luck finding a solution!