In the end the unsuccessful missionary changed the world. The one convert he had became a prophet and taught others. Eventually he led the church to a spiritual revival. The unsuccessful missionary was Abinadi, and his one convert Alma the Elder, one of the most influential men in the history of the Nephite nation. No one I know thinks of Abinadi as unsuccessful or a failure nor should we.
When we get tired or discouraged, when we wonder if we are doing any good, we can think of Abinadi. Who knows what effect you will have on people or how exactly that will manifest itself?
I met a man when I served a mission. He had been married to an active member of the church for over 20 years and had gone through many sets of missionaries, My first visit was where he had said for the first time that he believed in the church. He was baptized a short time later. I did precious little to change his heart. When he talked about it he talked about great members of the church that had been stellar examples and had worked with him many years previous to this. They planted the seeds, and I was lucky enough to see the seed grow.
Planting is more important, so don't forget to sow the seeds even if you are not around to see the plants. Don't get discouraged if you don't see the plants because they will come in time.