Make sure after every point you play to check in to see where your mindset is, this will help you to be aware of your emotions and keep them in check better. Things can really snowball when you are playing a match, so we need to monitor our thoughts all the time on the court(also off it!). My advice is stop it before it starts, so you have to FEEL a sense of control when you are playing, this give you the confidents to handle your nerves on those big points! Because that will be the deciding factor weather you win or lose that tight match, it boils down to who can step up first and swing the bat, you wont be successful all the time, but you will never succeed if you don’t swing my friend. Like everything else I teach, start working on this in practice to make it a habit, after every point, check in to see why you won or lost the last point, then LET GO of it emotionally, this letting go is vital folks if you want to play the next point with a clear mind, this whole process take times, just go slow and stick with it, then watch your ability to stay in your matches improve.