Tearing up in front of people, during a performance or an argument can be frustrating, make one feel weak and powerless. How to overcome this?
Well, first of all, please have in mind that crying is a natural body reaction in order "clean" itself. It aslo helps to "clean" your mind and heart of bad emotions, pain, stress... So before you try to supress your tears, always make sure to ask yourself whether "supression" is really what you want. Sometimes it's good to just cry it out, because supressed emotions stay inside and can turn into something more serious over time if they are left unattended. So first rule - be kind to yourself.
However, if you do find yourself in a situation where you need to get yourself together and stay calm, here are a few methods how to do it.
1) Deeply breathe in, look upwards and blink a few times, then exhale all of it, together with the feelings that triggered the tears. During this, keep telling yourself "I'm strong. I won't cry over something like this." Personally, I find this one the most helpful.
2) Find something to laugh about. A funny video, a joke, a weird situation... You can even try to gently mock yourself or the situation, but beware - don't start bullying yourself!
3) Occupy your mind with a task and give it all of your undivided attention. If you can't think of anything, don't panic, but take your time and lead your mind to "safe waters". This method is avoiding and ignoring your emotions, which is why I suggest doing it only if you really must and when you feel like you are ready, face the emotions that you had supressed.
These are my suggestions, I hope it helped. Stay strong, you are not alone! :)