However, this doesn't mean that God won't use us to work in the hearts of man! He can and does work through people!
1 Thess. 5:11
"Therefore encourage one another and build each other up..."
"...how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them?"
The important thing is to preach, encourage, exhort and teach with His strength and His leading, not our own. Our own strength and leading only results in more disappointment and frustration. Apart from Him we can do nothing of eternal value. He is the One that causes the change, not us. The Holy Spirit is the One who convicts, not us.
“I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing." John 15:5
"When he [the Holy Spirit] comes, he will convict the world, and show where right and wrong and judgment lie. He will convict them of wrong..." John 16:8
I think Paul explains it well in 1 Cor. 6:6-7 "I planted, Apollos watered, BUT GOD was causing the growth. So then neither the one who plants nor the one who waters is anything, BUT GOD who causes the growth.…"
Quit TRYING to make your husband someone he isn't. Quit bossing your sister around because she isn't living the way you want. Quit nagging at your friend for her decisions. What would happen if we prayed for people as much as we talk at or about people? Do you change by the force of others around you? Is it not through a surrender to Christ and His power in your life? That is what brings change in your own life and in every other person's life as well. Leave the business of changing lives up to God. Allow Him to use you as He wills and leave the results up to Him.
p.s. as always, preaching to myself here :)