That God is good is a fundamental of the practice of faith.
We might believe in all the things that we learn about God. But until we have internalized the idea of God’s goodness — that God is “The Good One Who Is Doing Only Good” (“…Ha-Tov v’Ha-Meitiv”/הטוב והמטיב), as the brachah says — we will have only the mental calm of “belief;” not the full-hearted joy of life that “faith” gives us.
As Rebbe Nachman of Breslav says:
“It is very good to rely on God completely. As each day begins, entrust your every movement and those of all who depend on you into God’s hands, asking that everything should go according to His will.
You will then not need to worry about whether or not things are going as they should, because you are relying on God. If He wants things to go differently from the way you may wish, you will be willing to accept everything the way He wants it.” 
I’m not sure why the Rambam didn’t include it in his “13 Principles of Faith,” although we might find it inferred there.
But whether or not he stated it explicitly, experience verifies that the practice of faith requires it.
God is our “spiritual environment.” The milieu, or “matrix” in which we live; more than live, in which we exist at all. Just as the water of the ocean is the “milieu” in which a wave lives and moves; just as the sun is the ongoing source of every ray of sunlight — so is our life and existence an expression of God’s.
But unlike “water,” God — our spiritual environment — responds in kind to our thoughts and actions.
Because of their sense of living with and in God, Adam and Chavah began their lives with a profound, tranquil feeling of security until eating a fruit they had been told not to eat. They didn’t have a single second of fear, worry, shame or guilt until they did what God told them not to do. They created their own sense of separation from God after which God — always kind and loving — seemed to them to be fearsome and punishing.
We correct their mistake by choosing to turn our attention to God’s real Presence and Goodness:
“Thou keepest him
in perfect peace,
whose mind is stayed on Thee,
because he trusteth in Thee.” 
(I prefer the King James English here, over any modern English version. The poetic rhythm is so much better!)
Yishiyahu is telling us not only that trust in God brings peace, but that trusting in God means always directing our attention to God: “…his mind is kept on You because he trusts You.” This is what sacrifice, worship, meditation or prayer should be; are meant to be.
We must learn to do this. But how?
One way is to take ideas about God — especially about God’s Goodness — and make them so familiar to our minds that they are always easily recalled. As Wordsworth said of mentally recalling the daffodils he saw:
For oft, when on my couch I lie
In vacant or in pensive mood,
They flash upon that inward eye
Which is the bliss of solitude;
And then my heart with pleasure fills,
And dances with the daffodils. 
Wordsworth shows us here that his mind and heart respond to the mere thought or mental image of the daffodils in the same way they responded to the physical sight of those flowers.
The principle holds equally true of God’s Goodness: If we can, “in vacant or in pensive mood,” [i.e. with a relaxed mind; without mental effort] allow God’s Goodness to “flash upon [our] inward eye,” without distraction (what else is “the bliss of solitude?”), then our hearts should be ‘with pleasure filled.’
But to do so, we must create an image in our minds of what that Goodness means in our lives.
As Rabbi Lichtenstein and other teachers of visualization and spiritual healing teach: Take your attention off any thought of the problem itself, and place it only on an image of the outcome of the problem that best expresses God’s Goodness.
We must do this in “vacant or pensive mood” — i.e. in a state of mental and physical relaxation.
We must not think that we are creating the outcome ourselves, at all. It is God Who creates the outcome, not us. We take the image, and allow God to be God. We mentally observe the outcome; we “witness” it.
If we are praying liturgical prayer — speaking the prepared, set words — we can also “keep our mind on God” by mentally seeing only the positive outcome, for ourselves and everyone else, of God’s Goodness expressed in the world.
No “force” is required.
That is how we can “practice” faith. Our mastery of it can be gradual and incremental; we need not look on it as something we either “do or don’t do.”
It’s something we can develop step by step.
 Rebbe Nachman of Breslav: Sichos Ha-Ran # 2
 Yishiyahu/Isaiah 26:3
 Wordsworth, William; I Wandered Lonely As a Cloud (1815)