I'm going to assume--maybe wrongly--that you're not too worried about the happier emotions, such as happiness, joy, enthusiasm, and so on?

As for the rest, just about all negative emotions can be classified under two headings: aversion and attachment.

Aversion includes anger, frustration, irritability, jealousy as well as fear and some forms of anxiety  (I assume that some will try to argue that jealousy comes from attachment, but I'm talking about the actual phenomenology of jealousy, which is to control or to attempt to change something that's really none of one's business, which is a form of aversion to things as they are.)  

Attachment includes neediness, lustfulness, attachment to craving, attachment to one's desires (not desires themselves but the attachment to them), clinginess, and certain forms of anxiety, among other mind states.

So, how do you prevent yourself from being too emotional in a given situation?

You learn to work with those two categories of emotions outside of those situations.

Think of sports.  

If this baseball player had waited until his at-bat to figure out how to hit a ball, he'd be in trouble.  Instead, he practices and practices countless hours before he steps up to the plate.  It's the same in any sport, any artistic performance, and so on.  

So, it's similar with emotions.  If you're waiting until those given situations, it's a bit late.  You want to find a way to "practice" with your emotions before those "moments of truth."  

So suppose that your troublesome emotion is anger or rage.  Then, learn to spot weaker forms of aversion in your daily life, such as when you feel mildly irritated or annoyed.  Don't just let that go thinking, it's okay for me to be irritated or annoyed.  

Instead, think of it as an opportunity to learn to handle aversion better. So, try to work with that irritation or annoyance, such as through writing about your feelings, or learning a little meditation and being able to sit with those uncomfortable feelings.  Or talk to people that you trust about these milder feelings.  

The same goes for attachment. So, if your problem is that you get clingy, say, in romantic relationships, try to look for how you tend to get attached or needy in milder ways, and learn to work with that.  Don't suppress it, and don't just say, It's okay that I'm this way.  

Instead, find a way to work with it skillfully.  

As you get better with the weaker forms of aversion and attachment, you'll accomplish two things: (1) you'll be relieving the internal pressures within you so that they don't build up so much to the stronger forms of aversion and attachment and (2) even when the stronger forms show up (because they still will), you'll have learned better how to work with those difficult emotions.

So practice with the milder forms of your difficult emotions outside of the given situation.  That's the summary sentence.  Hope this helps.