Isaiah 40:28

Hast thou not known? hast thou not heard, that the everlasting God, the Lord , the Creator of the ends of the earth, fainteth not, neither is weary? He giveth power to the faint; and to them that have no might he increaseth strength.—Fear thou not; for I am with thee: be not dismayed; for I am thy God: I will strengthen thee; yea, I will help thee; yea, I will uphold thee with the right hand of my righteousness.—Thou hast been a strength to the poor, a strength to the needy in distress, a refuge from the storm, a shadow from the heat, when the blast of the terrible ones is as a storm against the wall.

The trying of your faith worketh patience. But let patience have her perfect work, that ye may be perfect and entire, wanting nothing.—Cast not away therefore your confidence, which hath great recompence of reward. For ye have need of patience, that, after ye have done the will of God, ye might receive the promise.

Exhaustion means that the vital forces are worn right out. Spiritual exhaustion never comes through sin but only through service, and whether or not you are exhausted will depend upon where you get your supplies.

Jesus said to Peter - "Feed My sheep," but He gave him nothing to feed them with. The process of being made broken bread and poured out wine means that you have to be the nourishment for other souls until they learn to feed on God. They must drain you to the dregs.