I used cool mix for a while on a few of my horses. I noticed it did help as they were quite hot and energetic. I could just keep them under control better and at shows etc. they weren't as hard to handle or ride.
There is a feeding supplement called 'magic' which a lot of my friends use and apparently it is amazing?
Hope that helps. Definately recommend to give it a go if you are unsure..give it about 2 weeks before you give it a chance to make a difference. But depends how your horse is, there are quite a few exercises or tactics I used to use with my horses to calm them down, which worked great?
I've started using Super Calm by Global Herbs and it is incredibly effective. I only feed a calmer in the spring when the grass first comes through and this one is by far the best I've tried.