I would also suggest doing more ground work or having someone do it for you if you can’t right now. Also, look up T-Touch. It is a wonderful systerm that is very handy to know. There is a book on the method. I used it with my 8 year old Arabain geld. when I first got him. He was as hot a mess you could imagine coming from a previous owner who used a heavy hand. Took us a year, but we did a ton of ground work, proofing for trails, Clinton Anderson methods, clicker training, and TTouch. The T Touch was used to calm him down when he wouldn’t listen to my cues.
Another thing that helped me was to teach my horse HOW to calm down. I taught him to lower his head because when he got worked up he raised it like no tomorrow! I started on the ground with pressure/release at the poll and transitioned it to on the ground with a halter/lead then to bridal/reins. Basically a stretching excercise but it helped him gather his emotion.
I never have used a supplement for calming,so I can’t help along that line. I hope these tips may help along with your supplement. Good luck!