Frame Challenge: You Don't Really Need To
You want to prevent people from using emotional highs to consistently beat enemies and complete challenges? Look, as far as I know, you can't. The more intense the emotions you're feeling, the more stressed out you are and the less capable you are of thinking straight. Feel too many strong emotions at once, or just one really strong emotion, and you get overwhelmed and/or confused. Feel strong emotions for long enough, and you'll get drained.
Thus, any task that requires complexity or precision will actually be more difficult during an emotional high. Sure, you may be able to blast a door open instead of magically opening the lock, but it'll consume more energy and will likely get you caught by whoever's inside. And if you're in any state of mind that makes one want to blast a door off its hinges, chances you can't think clearly enough to do anything else.
Feeling tumultuous or confused? Cool, your magical energies will be disorganized, making your spells much less effective. So instead of flinging cohesive fireballs, you'll instead get fireballs that fall apart when thrown, sloppily streaking through the air and likely dissipating before they even reach the target. (Think of a cloud of ink in the water; cool-looking but not at all focused.)
What if you're feeling overwhelmed by the strength of your emotions? Well, sure, you'll have a lot of power running through your system-too much power. You know how Goku yells and glows when ramping up? Same visual, different mechanic. See, in this case, you're magically overloaded, which is painful as all get-out. You're going to scream, and your body is going to be beaten up, drained, and perhaps a little crispy afterwards.
Furthermore, feeling strong emotions over and over is taxing; your mages will get drained if they try that foolish strategy (relying on emotional highs to succeed) and thus won't be able to perform as well as they could have if they simply learned how to control their emotions.
Anyway, I hope this helps, sorry if it was a little too sarcastic.