During a time of discouragement due to not seeing anyone converted despite best efforts Jim Elliot says this, "It would be easy to slip into the business world and just be a good guy with a lot of religion rather than a producing son of God in enemy territory."
We all face the same type of discouragements if we truly seek to be used of God in bringing souls to Christ. Let us take heart that men of God in the past have face the same discouragements - but did not give into them. They kept praying and pressing in. Some of them (like Jim Elliot) did not see the fruit in their own lifetime. "Except a corn of wheat fall into the earth and die ..."
I believe God allows us to face these discouragements to make us realize that we cannot accomplish anything in our own strength. We must be brought to an end of ourselves (and this is usually a painful process), so that we can begin to experience the life and power of the Holy Spirit flowing through us bringing deliverance to the captives, sight to the blind, healing to the brokenhearted and LIFE to those who are DEAD in trespasses and sins.
Preaching the gospel requires the same sort of supernatural power attending to the spoken word as what was evident when Jesus said "Lazerus come forth!" Our commission is to raise the dead. That is why Paul said "our gospel came to you not simply with words, but also with power, with the Holy Spirit and with deep conviction". ONLY GOD can accomplish this!