How to instantly become calm and keep thoughts and emotions under control
Don't try to shut down thoughts and emotions - they're a natural part of us. Instead, do not get caught up on them. Experience but don't run after them.
A perfect example of being caught up by our thoughts is explained really well by the creator of the meditation app called Headspace: thoughts are like cars on the road - they come and go. Don't run after the cars; just watch them pass.
Besides, the thoughts will never run out either. They're generated indefinitely. The more you delve on the thoughts (thinking about thinking), the more of them will be generated. You can think yourself crazy this way.
No matter how much you think, in many cases, you can't think your way out. It's because some things can't be simply solved on a logical level.
Like anxiety - it doesn't matter how much you think - you'll never think your way out of anxiety. Most likely, you'll become more anxious.
Focus on the present moment and at the task at hand instead. Thinking about possible outcomes and possible future scenarios will just make things worse - too many thoughts will be generated.
Thinking should be a tool - to be used when it is needed.