I am nothing
Bitul (self-nullification). Learn to think "Who am I to want anything? We are not here for pleasure. I am doing the work of the king every time I say "No". What more can a Jew want in this world?
Along these lines of Bitul, R' Nachman offers a powerful suggestion. Whenever you are feeling weak and about to break, go into a room alone. Close your eyes, lie on the floor and put yourself in a place of total Bitul (self annulment). Repeat over and over again "I am nothing", "I am nothing".