I too served a mission, and it was very difficult. At my mission farewell, I remember my older brother, in his talk, telling me, "If you knew how hard this is going to be, you probably wouldn't go". And maybe I'm a Pollyanna, yet with all the hardship and difficulties, I can still say "It was the best two years" (1 1/2 years actually) of my life. I loved the people I served, even though most of them didn't get baptized. I loved my companions, even though we definitely had our differences. I loved my mission president, even though he was very opinionated with certain areas of the gospel. None of us are perfect. Our leaders aren't perfect. The Lord uses us as his tools--imperfect tools to further His work. Even if you only had one convert, (and that convert may be you, yourself), then your mission was successful. I commend all the missionaries who persevere and complete their missions. That in-of-itself is success. Missionaries are sent to serve. And sometimes, it may be a companion that you will be serving--helping them with their own struggles and weaknesses. And for me, one of the most difficult parts of my mission was learning, very clearly from my companions, just what my weaknesses were.
I remember a particularly very difficult time while on my mission. No need to go into particulars, but I was feeling so down, unloved, and with an overall feeling of not accomplishing anything. I was on my knees, and praying so hard for comfort and guidance. While I was praying, I literally felt someone give me a hug, but no one was in the room with me. After that experience, circumstances didn't get any better, but I knew that I wasn't alone, and I knew that the Lord was mindful of my joys and sorrows. There were other times when I prayed for help and guidance, but didn't receive that same sort of assurance. Sometimes, our faith needs to be strengthened. And the Lord will answer our prayers in the best way for us, not the way we particularly want them answered.
I came home from my mission with an absolute conviction and testimony of the truthfulness of the gospel. I wish I could say that with that testimony came a person who lived a perfect life. But, unfortunately I made and still make mistakes in my life. But, I know who my Savior is. I know where to turn for comfort. I know repentance and the atonement are real. That was the greatest blessing of my mission--my testimony. I wouldn't trade that for anything.