In looking around, I was interested to see that this comes from a homily by Cardinal Tarcisio Bertone who helped John XXIII organize and run the second Vatican council. He must have known the pope well and so this means all the more.
The daily decalogue of Pope John XXIII1) Only for today, I will seek to live the livelong day positively without wishing to solve the problems of my life all at once.
2) Only for today, I will take the greatest care of my appearance: I will dress modestly; I will not raise my voice; I will be courteous in my behaviour; I will not criticize anyone; I will not claim to improve or to discipline anyone except myself.
3) Only for today, I will be happy in the certainty that I was created to be happy, not only in the other world but also in this one.
4) Only for today, I will adapt to circumstances, without requiring all circumstances to be adapted to my own wishes.
5) Only for today, I will devote 10 minutes of my time to some good reading, remembering that just as food is necessary to the life of the body, so good reading is necessary to the life of the soul.
6) Only for today, I will do one good deed and not tell anyone about it.
7) Only for today, I will do at least one thing I do not like doing; and if my feelings are hurt, I will make sure that no one notices.
8) Only for today, I will make a plan for myself: I may not follow it to the letter, but I will make it. And I will be on guard against two evils: hastiness and indecision.
9) Only for today, I will firmly believe, despite appearances, that the good Providence of God cares for me as no one else who exists in this world.
10) Only for today, I will have no fears. In particular, I will not be afraid to enjoy what is beautiful and to believe in goodness. Indeed, for 12 hours I can certainly do what might cause me consternation were I to believe I had to do it all my life.