A lot of good suggestions and advice on this thread.
I wanted to share that I do use a "calming" supplement for my horse (pony)--Perfect Prep Training Day, which is a daily supplement (not the Perfect Prep paste products).
My TB/Welsh 4-going-on-5 gelding, who acts like a TB but shows his pony side through his easy keeping abilities, great feet, and smart a$$ ponytude, had some issues last year with riding in groups of horses. It's one of those issues you deal with when you raise a foal on a small farm and then typically ride him by himself. A friend recommended Perfect Prep and I've used it since then.
The key with a supplement is understanding that (1) they do not always work for your particular horse and (2) they don't make the underlying issues go away. I found that the supplement helped my guy focus and gave me a chance to ride him through the hinky moments without him completing forgetting I was on his back. However, he still has days (like yesterday in the freezing cold and coming back from 2 weeks off of work) where he's just full of it, marching to the beat of his own drum, and thinks he knows everything. :)
I personally feel that you really need to deep down understand what is driving your horse's behavior--turnout, feed, weather, etc, just play a role in exacerbating what is already going on in his mind and body. Is it pain (eg, ulcers)? Confidence issues? Is he getting fitter and healthier and figuring out how athletic he is?
Maybe I'm just odd :winkgrin: but I spend a lot of time observing our horses outside, in their stalls, and interacting with each other and other people. Each one is so completely different and requires a customized approach to feed, turnout, riding/driving, etc.
While my pony appears very confident, he is actually a bit insecure and uses his bravado to cover up his insecurity. All his training is built on increasing his confidence and keeping things simple on days when he's acting 2 instead of 4.
For example, if he has sparks shooting out, I may just get on and walk around; when he's walking calmly and flat footed, I stop for the day and get off. Sometimes the lesson starts when he's coming in from turnout and bouncing around. I make him stop when I say, back away when I walk toward him, stay at my shoulder at the walk...basically, I provide a framework of "acceptable" versus "unacceptable" behavior so the lines are clearly defined, but I work from the simplest steps up. If he doesn't respect me on the ground, he won't respect me under saddle; if he can't behave at the walk, how can I expect him to behave at the trot?
In addition, he's very smart, so our work is always varied--lots of playing with lateral work and transitions in the ring and hacking out as much as possible. I introduced him to fox hunting this winter and he adores it! He's learning that if I ask him to do something that (1) it is not an option and (2) I won't ask him to do anything he can't do.
Therefore, the stable management aspect is geared toward his personality and "type." As I mentioned, he's an easy keeper so he gets Triple Crown Lite and the only other supplement is PP. He's on grass hay that we make ourselves. He does get turnout every day, less in the winter due to shorter days and also less space as we use sacrifice paddocks. This is one of the reasons I keep him in some type of work during winter...because he gets so bored otherwise! Summers he goes out at night on grass...and my biggest challenge is managing his weight and making sure he gets enough sleep (he's not a morning pony...he likes his nap before a ride LOL)!
Our other ponies, well, like I said, they are all different. Our younger driving pony (6-year-old TB/Welsh) can be nervous and obviously lacked confidence when we began working with him. He likes a very consistent work schedule. When he's nervous, his "safety blanket" is a 20-meter circle and he stays there until he relaxes; then other things are slowly introduced. He's smart but not an overthinker, so there isn't the need to keep him as stimulated.
His other "security blanket" includes being in harness with blinkers, which keeps outside stimuli to a minimum. You can almost hear him sigh with relief when he's hooked to a carriage and my husband gets in and gathers the reins--he knows what to expect and feels "safe" in that setting. Riding is much more stressful for him, but cross training is part of our program and we continue to figure out how to reach that same level of relaxation that he has driving while he's under saddle.
He's also a harder keeper and prone to ulcers (after a tooth surgery that required large doses of NSAIDs while he was in the clinic). It took us a day or two to figure out what was wrong when he first developed them as he didn't show any classic ulcer symptoms...he just spent a lot of time running in the paddock and then had a meltdown under saddle. We finally had an "ah-ha" moment and started Gastrogard--new pony within two days. He'd been running "away" from the pain--the only way he could come up with to "deal" with the pain.
We feed him Ultium, which I've used with success in another ulcer-prone TB. It is very calorically dense so you don't have to feed a lot of concentrate. In addition, of all the feeds I compared, it had the best balance of low starch/sugar level, high fiber, high fat, and lower protein, among other things. He also gets the supplement UGard. Finally, while the first pony prefers single turnout, this pony is only happy with his two donkey friends.
I could go on and on, and already have, but I hope this provides some ideas and a different perspective. You may already "know" your horse, but do make sure you understand what is instigating his behavior. My first rule out is always pain, followed by innate personality/behavior, which dictates how they respond to pain, work, other horses, and their "humans." Once those are understood, the variables of stable management and riding can be adjusted to give the horse the best possibility of blossoming.
P.S. I'm a huge proponent of long lining, but only recommend it if you can have someone show you how to properly do it. I took lessons for 6 months before the whole concept truly clicked. It is not simply "driving" from the ground or lunging with two lines...the lines are used to imitate more than just your rein aids. If issues start cropping up while riding, I always go back to the long lines to reestablish a good connection and instill confidence.