What does the worker gain from his toil? I have seen the burden God has laid on men. He has made everything beautiful in its time. He has also set eternity in the hearts of men; yet they cannot fathom what God has done from beginning to end.
~Ecclesiastes 3:9-11~

Really, what do we gain from this work that we're doing?  What is it that we benefit from pursuing this burden of the lost that God has given us?  Some days it may seem like the answer is "nothing but heartache" or "only an increased burden."  The fact is that it's not about what we gain.  We've heard it over and over again: "It's not about YOU." It may seem trite but it's true.  No matter how much time you feel you've invested, no matter how easy it is to get tired and want to give up, we have to remeber that God makes everything beautiful in HIS TIME.  Not mine.  So many days I want to see where the end is.  But this verse says that it's not for us to know the beginning or the end.  We are only to do the eternal work that He has placed in our heart. 

He will take our efforts and make them beautiful.  If we are walking in His will, God will use the smallest thing to bring about beauty.  It may not look admirable to you or your friends, but you never know whose life will be touched.  He brings the harvest and makes us fruitful.  But we must be faithful to do our part.  There is a verse that follows after those I've noted.  It says:

I know that everything God does will endure forever;
nothing can be added to it and nothing taken from it.
God does it so that men will revere him.
~Ecclesiastes 3:14~

Are you letting Him do things through you?  Are you starting each day by saying "God let nothing I say or do interfere with your enduring work?"  It takes effort to remember that some days.  But take courage.  Look back on yesterday and see what God did. 
Revere Him. Love Him. Stand in Awe of Him. 
Keep doing the eternal work!