"I am weary," he [Hujjat] confessed, "of the grim resistance of this people. They are evidently animated by a spirit which no amount of encouragement from our sovereign can hope to call forth in our men. Such self-renunciation surely no one in the ranks of our army is able to manifest. No power that I can command is able to arouse my men from the slough of despair into which they have fallen. Whether they triumph or fail, these soldiers believe themselves doomed to eternal damnation.

Shoghi Effendi, The Dawn-Breakers, p. 570

Do not feel discouraged at the meagerness of the results you now obtain. The Master's promises regarding the share you are destined to contribute towards the spread of the Faith in the Far East will sooner or later be completely realized. No matter how dark the present may appear, you should feel nevertheless confident that the distant future is immeasurably bright. Strive, therefore, with a joyful radiant and confident heart to hasten the fulfillment of ‘Abdu’l-Bahá's glorious promises. Your reward is unimaginably great, and the success that awaits your labours certain.

Shoghi Effendi, Japan Will Turn Ablaze, p. 66

Do not feel discouraged if the work you are doing for His Cause does not bear rich and immediate fruit. The seeds you are so patiently and devotedly sowing will assuredly germinate, and future generations will reap an abundant harvest. The Master is watching over and blessing your historic services. Rest assured.

Shoghi Effendi, Japan Will Turn Ablaze, p. 65

Do not feel discouraged if your labours do not always yield an abundant fruitage. For a quick and rapidly-won success is not always the best and the most lasting. The harder you strive to attain your goal, the greater will be the confirmations of Bahá’u’lláh, and the more certain you can feel to attain success. Be cheerful, therefore, and exert yourself with full faith and confidence. For Bahá’u’lláh has promised His Divine assistance to everyone who arises with a pure and detached heart to spread His holy Word, even though he may be bereft of every human knowledge and capacity, and notwithstanding the forces of darkness and of opposition which may be arrayed against him. The goal is clear, the path safe and certain, and the assurances of Bahá’u’lláh as to the eventual success of our efforts quite emphatic. Let us keep firm, and whole-heartedly carry on the great work which He has entrusted into our hands.

Shoghi Effendi, Lights of Guidance, p. 580

He urges you not to allow yourself to be discouraged, as all these temporary conditions will pass away as the Faith grows, but to concentrate on the constructive work of teaching and exemplifying the Faith.

Shoghi Effendi, Lights of Guidance, p. 505

If we but turn our gaze to the high qualifications of the members of Bahá’í Assemblies . . . we are filled with feelings of unworthiness and dismay, and would feel truly disheartened but for the comforting thought that if we rise to play nobly our part every deficiency in our lives will be more than compensated by the all-conquering spirit of His grace and power.

Shoghi Effendi, Bahá’í Administration, p. 88

Let nothing frustrate or discourage you. Immerse yourselves in he study of the Holy Tablets. I testify before God, Bahá’u’lláh asserts, to the greatness, the inconceivable greatness of this Revelation. Again and again have We, in most of Our Tablets, borne witness to this truth, that mankind may be aroused from its heedlessness. In another connection this utterance has been revealed: How great, how very great is this Cause. The day is fast approaching when its stupendous glory will have been made manifest and evident.

Shoghi Effendi, Fire and Light, p. 33

Such pioneer work is always difficult and unless we are strong-hearted and have faith in our Cause, we are apt to be discouraged. We should always bear in mind that Bahá’u’lláh is always on the watch ready to come to our assistance if we only rise to serve Him with the Spirit of self consecration and absolute detachment. His promises along those lines are very forceful and clear, we are only to act upon them.

Shoghi Effendi, Lights of Guidance, p. 580

Also, don't be discouraged if your first efforts do not meet with success.

Universal House of Justice, Guidance for Bahá’í Radio, p. 55