...you learn how to control your emotions.
I read this quote recently, and while I understood the author's point, I think it's a misguided and misunderstood sentiment.
Controlling your emotions has nothing to do with control or emotions.
Emotions are a byproduct of how you're interpreting reality.
The question becomes, how much control do you have over how you interpret reality?
Well, you'd further have to investigate and ask how much control you had over the environment you grew up in? To which the answer is minimal.
People who display little emotional control usually grew up in environments where there was little emotional control; it's wholly learned behavior.
This doesn't mean there's no hope, and you should remain a victim of your past, it's just important to start with a compassionate stance that considers why people may have limited command of their feelings.
The truth is, when your identity is intact (when you know who you are apart from your experiences), there's no need to control emotions; they're inherently under control because there's no attachment to previous encounters.
Fluctuating emotions is a natural compensation of an identity crisis (when you define yourself by your past). Until you unravel your identity from the events of your past time, emotional stability is unlikely.
We need to stop chasing symptoms and better understand the mechanisms driving them.
"Your intellect may be confused, but your emotions will never lie to you." Roger Ebert