Did you check for ulcers? That’s the most common reason for a nervous horse, their stomach hurts. But then there are horses that are generally just more nervous, do you mean when riding?
This isn’t really a feed supplement but I read somewhere that if you take a little bit of lavender oil on your finger tips and rub the horse’s forehead in circular motions, it will calm them down.
Hey! I use Good as Gold from TRM! On the daily bases I give it in powder, in competition I use paste (not sure is the right word)! It's natural so it's not dopping and it really works, they stay more relaxed and focused on what really matters
I like perfect prep extreme fast working for a show for everyday use I would recommend smartcalm ultra pellets
I would try forecalm from equine forefront. Fallon Taylor uses it and it seems to work