I can understand the discouragement, but you're still doing the duty that we all commissioned to do here, regardless of the results, they are in God's hands. I've seen people immediately repent and have stayed strong in the faith for years since, and I've seen people who out right rejected it and then later find them participating/talking about their church to my surprise. I don't always know the success of my ministry, and I don't think it should always matter for us to know, but what I do know is that it is my duty to share this good news to those who haven't heard. The farmer must wait until they start to see the first blade and then the crop before he can begin to reap at harvesting time, and you know what? Sometimes God calls some of us to sow the good word in men's hearts and others to reap what they did not sow for God's kingdom, while there are still yet others who are called to water that which God has planted and made to bud. All of these may not see each other in the work, but they do it for God. You don't know if your words will stick with that person, this has surprised me several times. I remember telling some of my childhood friends I no longer talk to about my faith...and the next time we met years down the road they would bring it up out of nowhere saying, "I still couldn't get what you said out of my mind, it baffles me." Yes, this has happened to me on several accounts, but they aren't saved. Maybe God will open the door for someone else, maybe another Ray Comfort will step in and finish where I left off.
Another thing to note is that God is glorified regardless of your success. The reprobate who may never be saved still needs to hear the gospel, and God will be glorified in that you did your part in preaching it to all. This verse of God speaking to the prophet Ezekiel has always reminded me of that:
“Son of man, I have made you a watchman for the house of Israel. Whenever you hear a word from my mouth, you shall give them warning from me. If I say to the wicked, ‘You shall surely die,’ and you give him no warning, nor speak to warn the wicked from his wicked way, in order to save his life, that wicked person shall die for his iniquity, but his blood I will require at your hand. But if you warn the wicked, and he does not turn from his wickedness, or from his wicked way, he shall die for his iniquity, but you will have delivered your soul. Again, if a righteous person turns from his righteousness and commits injustice, and I lay a stumbling block before him, he shall die. Because you have not warned him, he shall die for his sin, and his righteous deeds that he has done shall not be remembered, but his blood I will require at your hand. But if you warn the righteous person not to sin, and he does not sin, he shall surely live, because he took warning, and you will have delivered your soul.” - Ezekiel 3:17-21
I always encourage to pray for people, even if you don't share the gospel personally with that person. Ask that he may send someone to them who will faithfully preach it and that God would open their hearts to receive it.