I wish I had had more realistic expectations.
Many girls and young women have unrealistic ideas of romantic bliss in marriage, without seeing the problems or day-to-day efforts needed to make a marriage work.
Missions is no different. In most missionary presentations, we hear only about the victories and great things the Lord is doing. I think that is for two reasons. First, missionaries are trying to recruit people into the work, so they strongly emphasize the positive side. Second, most people, and maybe especially missionaries, don't want to be vulnerable and reveal that they have problems. This is not helped by the fact that church people want to put missionaries on a super-spiritual pedestal for being willing to sacrifice and live under harsh conditions. So, it's important to talk with missionaries one-to-one to hear their struggles as well as their victories.
Another common fallacy is that the lost are crying out for someone to tell them the gospel. There may be the rare exception (in Papua New Guinea, one tribe did build a church in anticipation of the missionaries coming to tell them the good news), but in general, the lost are blinded just as the Jews are. They are not searching for God and are living deceived in the darkness of their blinded condition. We will most commonly find indifference to the message, and at worst, we'll experience downright opposition.
Missionaries who think they're going overseas to do a great work for Jesus amuse me. First, we merely participate in the work God is already doing. This is well explained in the book Experiencing God, by Henry Blackaby. Of course, all our friends at home will tell us what a great and wonderful thing we're doing. Then WHAM! We come face-to-face overseas with all our inadequacies and weaknesses. We realize how much we’re actually going to have to depend on God to see something accomplished. Many missionaries, when confronted with the reality of living in a foreign culture and the time needed to influence people, simply become discouraged, turn around, and come home. Only when we realize our total dependence on God, wait on him, and work with him do we finally see some beautiful fruit.
I wish I had known how difficult missions really is.
I would have gotten more cross-cultural training, especially focused on the culture to which I was going. I would have taken more time in language learning. But most of all, I needed realistic expectations. Working in a foreign field is the same as being in a war. I know. I've fought in both and the similarities are striking. There is not much glorious about warfare. It may look exciting on TV or in the movies, but in the trenches it's a lot of hard work. And the enemy has ambushes everywhere. Often you can't tell the enemy from the friendly. And your friends get injured and killed. It hurts.
The culture won't make a bit of sense and you'll even resent the people sometimes, or think how they do things is ridiculous. But you will learn how to live there. You'll learn new cultural cues. You'll begin to see how they do make sense in your new culture. And in the learning, you'll grow to love the people. So learn to laugh at yourself!
Don't give up! When you go, determine that you're going to stay. It's like God meant marriage to be. It won't always be easy, but make it work! Don't expect the other person to change. Change as you need to. And there's probably no better environment to promote change in us than working in another culture.