Rabbi Morris Lichtenstein wrote his textbook, Jewish Science and Health,  about 90 years ago. In it, his teachings echo Torah, Talmud and Hasidut. By it, he created a devotional classic that is no less relevant and inspiring today than when it was new.
Packaging may change over time. Dust jackets might be more colorful than before. Prose might be terser; simpler. Contemporary inspirational writing might employ shorter paragraphs and use more “maxims” — short, easily-remembered bits of advice and insight.
But the necessary content doesn’t change.
Our problems are the same: Worry (in particular), fear, anger, sadness, envy, illness, (perceived) lack, etc.
If the problems are the same, the solution is the same, as well. For Rabbi Lichtenstein, this is emunah: Faith.
“A [person] of faith cannot worry.” 
“Faith in G-d…is given as the antidote to fear…” 
“One who is fortified with faith, cannot become the prey of grief…” 
How do we have “faith?” If we haven’t had it before, how do we start? If we have employed it before, but are now faced with new difficulties, how do we re-invoke it?
Having faith requires reorienting our most basic view of G-d.
G-d isn’t merely a “force” outside of Creation that/Who occasionally intervenes.
G-d is the ongoing, moment-to-moment Source of Creation; the essential “ingredient” of every animate and inanimate thing, from the cosmic to the infinitesimal. Just as water is the essential “ingredient” of every wave and current in the ocean, G-d, or G-d’s existence, must remain in created things:
“G-d must be conceived as the vitalizing force of all reality, as the essence of all existence.” 
Nothing can be separate from G-d, because G-d’s existence is the source of the existence of all else, as the Rambam says:
“The foundation of all foundations and the pillar of wisdom is to know that there is a Primary Being who brought into being all existence. All the beings of the heavens, the earth, and what is between them came into existence only from the truth of His being.
If one would imagine that He does not exist, no other being could possibly exist.” 
Rabbi Lichtenstein states this same idea repeatedly, in his characteristically straightforward, lucid way. Yet, what the Rambam calls “…the pillar of wisdom…” is also the centerpiece of Jnana yoga and of all Kabbalistic and Hasidic learning and contemplation, especially Habad “Hitbonenut.” We must take this idea, consider its implications and internalize it until it becomes our own view of all that exists and of all that happens to us and around us. Other sources (e.g. Tanya; Zohar) go into this in far greater detail and depth, but these details can make the theme far less accessible (at first). Rabbi Lichtenstein cuts to the heart of the matter in the simplest possible way:
“He is the soul of the world and permeates every particle of its substance.” 
Our eventual deduction must be that if G-d remains in all things created, G-d is present within us now as an integral part of ourselves:
“He is therefore the soul of every being; He is the very soul of man [and woman].” 
While this realization might first come to us as an intellectual conclusion, it must ultimately be more: It must become a heartfelt, personal experience:
“This faith must be genuine; it must be felt as sincerely as it was held by the Psalmist [King David].” 
If “M’lo kal ha’aretz k’vodo”  — if the whole world, the whole creation, is filled with G-d’s glory, G-d’s Presence, then we must look upon ourselves as equally filled with that Presence, that Light, as well.
G-d is “in” us as the essence of what we are. We can never be separate from G-d, any more than a wave can be separate from the ocean’s water.
Because G-d is “in” us, we have access to all that G-d is; to all that G-d can give:
“G-d dwells in man. His Presence is a tower of strength and a shield to guard him from all ill. Man must realize this truth, and the realization of it will drive out all fear.” 
We access this Divine aspect of ourselves through prayer. We especially draw on it through “visualized,” affirmative prayer. The “human” level of our mind initially forms the image, but the Divine level creates the outcome, to the extent that we allow it to do so.
Again, this “realization” isn’t simply intellectual assent to an idea. It’s a heartfelt change in perspective, brought about by the contemplation of the true relation of G-d to the “world,” and applied in our lives, especially in prayer:
“…G-d cannot be perceived through the mind [i.e. intellect] alone. If you would know G-d, do not seek merely to prove His existence, but turn to Him with your heart; affirm your union with Him, affirm His responsiveness to prayer, pray to Him; if you actually turn to G-d … speak to Him in your heart, you will be astonished to find how close He is to you, you will feel His nearness, you will have found G-d.” 
When Rabbi and Mrs. Lichtenstein talk of “prayer,” they mean “Visualization” or “Affirmation.” Rebbe Nachman talks of “Hitbodedut,” but our understanding of “What” and “Who” we are talking with deeply affects our practice and our progress, whatever method we use; whatever path we follow.
Every prayer is an experiment!
Prayer that’s not based on a perspective in which G-d fills everything as its very essence — fills us as our very essence — will seem to work out of some “magic” or “mystery,” if it works at all, rather than reveal, as Mrs. Lichtenstein writes, G-d’s actual nearness to us and loving activity in our lives.
 Lichtenstein, Rabbi Morris; Jewish Science and Health; © 1925
 ibid., p. 186
 ibid., p. 204
 ibid., p. 207
 ibid., p. 204
 Maimonides; Mishneh Torah; ch. 1
 JS&H, p. 204
 Isa. 6:3
 JS&H, p. 204
 Lichtenstein, Tehillah; Applied Judaism; Doris Freedman, ed.; “Can We Prove That G-d Exists?”; p. 96;
(originally part of “How Shall We Find G-d;” Jewish Science Interpreter, June, 1940; p. 4)