First, know that God's word has not failed in its task, and therefore you can know that you have not failed. No one's salvation is dependent upon your ability, but only on God's mercy. God's mercy never fails, for whoever has been set apart for salvation, chosen by the grace of God, will be united to Christ. This is why we depend on the power of God in his word, not on our own words. This is also why someone like Isaiah could prophesy when God had sent him with a message that made it evident that his would be a fruitless ministry as far as he would be able to see:
And I heard the voice of the Lord saying, “Whom shall I send, and who will go for us?” Then I said, “Here am I! Send me.” And he said, “Go, and say to this people:Thus, our continuing in ministry, whether official or simply living as salt and light in the world, ought not be founded in the sort of success which we are used to thinking of as success. Nor should we be unduly distressed by this sort of failure. It is not failure, either our own or God's. Rather, it is God doing as he wills.
“‘Keep on hearing, but do not understand;
keep on seeing, but do not perceive.’
Make the heart of this people dull,
and their ears heavy,
and blind their eyes;
lest they see with their eyes,
and hear with their ears,
and understand with their hearts,
and turn and be healed.”
Then I said, “How long, O Lord?”
And he said:
“Until cities lie waste
and houses without people,
and the land is a desolate waste,
and the Lord removes people far away,
and the forsaken places are many in the midst of the land.
And though a tenth remain in it,
it will be burned again,
like a terebinth or an oak,
whose stump remains
when it is felled.”
The holy seed is its stump.--Isaiah 6:8-13
This is not to say that we ought not be sad and mourn for the lost, even Christ wept for Jerusalem, and he is not willing that any should perish. Yet, with great sorrow, some are set apart for destruction so that God's justice might be poured out on them to his glory. If no one was condemned, then it would appear as if we had some kind of right to salvation, and what would we then be saved from? Thus, in their punishment, we shall see how great God's mercy toward us is, since we shall see how great a punishment our sin deserves, and thus how great a grace he has shown to us in taking that sin, and thus that punishment, upon himself. We ought to desire, as does Christ, the salvation of all, yet be glad of the final judgment insofar as it constitutes the judgment of sin, which these people would not give up to Christ, but rather held onto their sin, and therefore bear the punishment which goes with it.
We ought not be glad that they rejected Christ our savior, but we ought to be glad that, because they did reject him, we will be able to see in their suffering how great the mercies of God to us are, and how great the sin we have been saved from is. It is not for us to know who is to be saved, but only to know that God in his wisdom has already chosen, and therefore that we can preach without fear of failure, since it is God who saves, through the preaching of the word. Therefore we ought to preach with great zeal, since we preach without fear of failure, but only with the hope that by our feeble words, which so often do not express the true message of the gospel well, some might be saved. It is not we who save, but Christ saves through us, and he is not dependent upon us, yet he uses us, that he might show how his power is made perfect in weakness.
And Jesus came and said to them, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” --Matthew 28:18-20Thus it is because Jesus has all authority, and it is because he will be with us always, that we are able to go and make disciples. It is not because we are able to, by our zeal or wisdom or other such nonsense, do these things, but because we are in Christ and he is able to do these things even through such foolish and weak people as we are. This is why we seek to preach the good news to all that some might be saved.
It is indeed true that we preach something without which all are dead, and this is fuel for preaching, but if it were merely by our power that this raised the dead (which is a power that no human has on his own) then we would become frustrated by how many it did not raise, and thus fall into despair, and preach no more, for our flesh is weak. Yet, because it is the Word of God which raises the dead, and not we ourselves, we continue to preach to the lost, in the hope that they might hear and believe by the power of God who is able to raise the dead and open the eyes of the blind. We preach, not to save lives, but to see lives saved by our savior and our God, Christ our Lord. Thus the zeal comes, not from the urgency of hell, but from the urgency of God who is coming soon. We seek to spread the word that all might hear and believe. The zeal, the urgency, is good. It ought not depend upon hell, though it is an image which shows us how great our zeal should be. Instead, our zeal is for God, who is worthy to receive glory and honor. We desire that others should be joined to Christ our joy, that his glory might be revealed to the fullest. Preaching with this kind of zeal is like opening a present excitedly: we do so, not in order that the present might be more likely to be what we want, but because we are already expecting that it will be a gift which we will be thankful for.