I really feel what holds players back, is their inability to recover after a mistake, players often stay in the past during a match, instead of recovering quickly and getting ready for the next point, which is the most important point in every match. Learn to let go of your mistake and stay focus by staying in the moment. This should help you from getting down on yourself, also never lose two point in a row, because tennis is a feel game and momentum plays such a big part in it, never allow your opponent to get any type a rhythm in the match. This should frustrate them, then turn up the pressure on them, to break their willpower!, You will never play perfect tennis , so don’t try to, just keep it steady as you can. Fight through your mistake by never getting upset after a mistake, this give you a sense of self control, that is very important for winning. I guess to sum it up , your ability to refocus, will determine the outcome of your matches. So, master this element of your game today, practice makes perfect, but give yourself time to master it. no need to rush, take your time. It will all come together if you are patience.