"I have labored to no purpose; I have spent my strength in vain and for nothing..." (Isaiah 49:4).
It is a startling fact that the Servant of the Lord, our Lord Jesus—who was made like us—had moments of bitter disappointment. This is one of the many ways in which he, in his humanity, became "acquainted" with our suffering and grief.
Some suggest that these thoughts arose in Gethsemane, but I think moments of discouragement were woven throughout the warp and woof of Jesus' ministry, and were the result of continual opposition, rejection, denial and betrayal. What was diffused throughout his life is condensed in this one verse: "I have labored to no purpose; I have spent my strength in vain and for nothing..."
"Nevertheless," the Servant insists, "my judgment is with the Lord, and my work is with my God (49:4)" It is for the Lord alone to judge the success and significance of his work. Furthermore, he is in partnership with his God and thus, in the end, he cannot fail!
So, the solution to our discouragement is to "spend our strength," but to look away from the outcome to the one who is faithful to perfect his work in due time. We may not see what our Lord is doing in our lifetime, but we can know with assurance that our labor is not in vain.
There's a lovely tailpiece to this text:
Indeed (God) has said,
"It is too small a thing that
You should be My Servant
To raise up the tribes of Jacob,
And to restore the preserved ones of Israel;
I will also give You as a light to the Gentiles,
That You should be My salvation to the ends of the earth" (Isaiah 49:6).
In other words, "You ain't seen nothin' yet!"
 Cf., Mark 8:21; 14:27; 14:50; Luke 9:41; John 13:21.
 The strong adversative with which this verse begins "But indeed!" emphatically counters what has preceded.
 Jesus did not see this promise fulfilled in his lifetime here on earth. Fulfillment was later and through other hands. So it may be for you and for me.