Look to nothing else, even for a little while, except to reason.
— Marcus Aurelius, Meditations Book I, Farquharson translation
Emotions are like stallions. When untamed, they roam without purpose. They travel in groups, and the slightest disturbance will leave the entire band galloping. They are extremely dangerous, but useful when tamed.
In order to tame our emotions, we must understand what they are.
Emotions are subconscious reactions to events and thoughts. They are very sensitive. A slight difference in temperature, a bit of light, or a few minutes of exercise can all separate sadness from ecstasy.
We care about emotions because they spur action. Emotional states strongly motivate against and towards doing certain actions.
Don't forget: thought is also an action. Sometimes, emotions lead to thoughts that bring about new emotions, making feedback loops.
Our actions today determine our future fate, so anything that creates action is extremely powerful.
It's also dangerous. Action motivated by emotion is probably irrational since when you're emotional, you don't care about rationality. If we can, we should avoid choosing actions based on emotions as much as possible.
What about when you've already rationally determined what the best action to do is, but are too unmotivated to follow through? It is in these cases that emotion can be useful. If you are emotionally tied to following through with the task at hand, you're more likely to be successful.
Therefore, we want to emphasize emotion when it motivates us to do useful things and de-emphasize it in all other cases.
How can we do this?
Most people recognize that limiting neuroticism is useful, so cultures around the world have developed techniques for this.
The naïve solution is repression. It doesn't work: it's like trying to put out a fire by punching it. Solving a problem like this with brute force is bound to lead to psychological damage.
Instead of repression, we want to diminish emotion by zooming out. This means shifting your frame of reference to encompass more of the universe. The zoomed-out mind ignores the petty triumphs and tragedies of the everyday. It instead contemplates the mountains, then the rivers and seas, outward to the planets and stars, and finally to galaxies and the universe.
Facing this grandeur, everyday events and thoughts become inconsequential. How does your crisis compare to the utter disintegration during the Bronze Age collapse? What is your joy compared to the stupendous beauty of the Pillars of Creation?
The more you zoom out, the tinier everything else seems. When scaled against infinity, everything finite shrinks to zero.
Children do this as they mature into adults. When you are a child, something like dropping your ice cream on the ground feels like a big deal. When you grow up, various experiences show you that things like that don't really matter. This is why children are more emotional than adults: they are naturally less zoomed out.
We can do this with adult worries, too, such that things like losing status or being in danger no longer incite irrational emotion. One goal of Hinduism is to bring this to the absolute extreme, such that absolutely everything that is not infinite (God) feels like nothing at all.
Start by visually "zooming out." Take your mind's camera and pan it backward, centered on your body. You will see the space around you, then your immediate surroundings, then your distant surroundings, the region, the continent... Stop when your mind boggles and the feeling of emotional grandeur no longer scales with the size of your scope. Keep practicing this, and you will go farther and farther with each attempt.
You can also do this temporally: start with the present and expand outward into the past and future (you can't predict the future's exact details, but science teaches us the general outlines). A linear scale is too slow: try powers of ten instead: a day, a fortnight, a year, a decade, a century, a millennium... once again, keep going until the grandeur becomes incomprehensible.
I've noticed that the more I practice this, the more zoomed out my default state of mind becomes. I've gotten to the point where I am better at tolerating misfortune than anyone I know. However, this works both ways: it is very difficult for me to become extremely excited or overjoyed by anything. I'm okay with this; make sure you are, too, before beginning this practice.
Recently, I've also realized that it's also possible to do the opposite: you can emphasize emotion by "zooming in" to a certain topic.
Stop thinking about anything but the task at hand: shut your mind out to everything else. Divide the task into subunits, then divide those subunits in turn as much as you can, then realize the immense importance of each part of the task. Analyze a topic to its very depths, and you will not fail to find immense importance in it, no matter what it is.
This is much more difficult for me than zooming out: I am not well-practiced. However, if successful, it makes it very easy to inculcate focus and get something done quickly. In counterpoint to the philosophy texts mentioned earlier, texts from Abrahamic religion seem good at reaching this type of frame of mind: the Qu'ran is particularly good.
In amplifying and diminishing our emotions, we become capable of using them as tools rather than allowing them to use us. In doing this, you bridle the stallion.