I don’t have any suggestions for calming supplements, but my mare was pretty hot and reactive when I got her. Nothing dangerous, but she could be a handful to turn in and out, and she was spooky/tense and quite reactive to sounds or things out of the ordinary. She would lose her mind if anyone touched a rail in the arena, so I hated flatting during jumping lessons.
She is now a different horse, is relaxed, and not very reactive at all, IMO. People comment on the transformation a lot.
Because so many things happened at once, it’s hard to pinpoint what helped the most, but the first thing we did that made a difference was putting her on Regumate. It’s pricey and you have to handle with caution (I do the compounded injections to minimize risk to barn staff and self). I know others have had success with Mare Magic, but I haven’t tried it.
If that hadn’t worked, we were going to scope her for ulcers, but the change happened very quickly.
It also turned out that she is asthmatic, so treating that and moving her to a shedrow and soaking her hay and increasing turnout also seems to have helped a lot too. She is VERY happy in her new set up, is not as fearful of barn cats and general goings on as she was before. I think she likes to watch the world go by!
I also think my energy fed into things. I don’t react to her anymore, so she trusts me and feels safe.
Long story short: she is now workman like and gets going to jump, but isn’t tense or anxious or fresh as a rule. Has her moments, but they are far between.