We have used Mare Magic, which is just cut and sifted Raspberry Leaf Tea. We now get it from an herbal medicine store.
You feed a scoop, about 1/2 oz i think about 15 minutes before you start working. It lasts for a while, like an hour or so but it gives you time to get started with a calm feeling so they an learn to experience that.
It does not leave your horse stupid or lethargic. It seems to work much like chamomile would for humans.
It works as a working calmer for geldings but is best as a calmer for moody mares, especially when in season.
It is not expensive and has no side effects so it is worth a try.
We just feed it straight. Our horses like it fine without having to mix it into anything, though it mixes with breakfast just fine, too.
I hope it works for you.