Be of good courage, and He shall strengthen your heart, all ye that hope in the Lord. -Psalm 31:24

Most men are subject to fainting fits at times. Even David became weak and faint; and Samson, after he had cried exultingly, “With the jawbone of an ass, heaps upon heaps, with the jaw of an ass, have I slain a thousand men,” yet, for want of a draught of water, was ready to lie down, and faint and die. The best of men are but men at the best; and, therefore, who wonders if their heart sometimes faileth them in the day of suffering, in the hour of battle, or under the broiling sun, when they are laboring for their Lord?

…He who fights most valiantly may be on the verge of victory, and yet be defeated, if his heart should then fail him. I have no doubt, in reading the records of many campaigns, you must have noticed that men have gone on from victory to victory, and suddenly there has been a pause because their hearts failed them, just when, had they followed up their previous successes, they must have swept all before them. Beware, you who have served God with courage, lest fear should take hold upon you, and you should flinch in the day of battle, and miss that which you might have won for your Lord.

…Oh, do not give way! You need not be cowards; do not give way. Do not say, “I must be beaten, I must always be despondent, my life is crushed.” You need not be so. “Be of good courage, and He shall strengthen your heart.” Get you to your chamber, fall upon your knees, pour out your heart before God, tell your trouble to the Most High, and, as the Lord liveth, before whom I stand, He must and will help those who put their trust in Him. ~ C.H. Spurgeon

http://www.spurgeon.org/sermons/2455.htm