Believe in you , but i mean , really believe in you , not just repeting it in your head , gotta feel it , also have fun while playing , but not smiling kindda fun , fun by impressing your self in trying some shots or whatever , and also never give up , also dont try to have *conversation* with yourself while playing if you know what i mean , example : gotta break point : '' ok now i have to win this point and then i can win the 2nd set and just one set to go blablabla...'' fuck that just play the point.
It's actually hard for me to describe it since im a very mentallly strong player , its in you , gotta find it by yourself.
I've to tell you thanks!
After I tried to play like you said...I became very strong...it's not why I'm better in technique,when I play a match I like how I stay on the court,very positive,always focused on the point,try to win all the points,try to keep the ball in(as it is my game,I'm not so tall as I said and I like to run and keep the ball in,waiting the right shot to make my opponent fail)...I never do more than 5-6 double faults(before I was doing double fault on the 50% or more of my second serve) and I play well on break point,without any fear! I'm very happy,cause now I'm also mentally strong and it's very important for me!