In my time studying applied linguistics, certain factors in second language acquisition have really stood out to me as key to success (or failure) in learning a language. One idea that really struck me is called attribution theory.
Anyone who’s ever seriously attempted another language knows that it’s not just a matter of showing up for class and absorbing material. There is a whole host of cultural, pedagogical, and personal factors that come into play; we might not even be cognizant of some of them. Attribution theory in general describes how humans explain things that happen in the world. In language learning, it describes how learners perceive their relative accomplishments or disappointments.
Psychologists divide up possible attributions into four categories: ability, effort, perceived difficulty of a task, and luck. Two of these factors are external and largely uncontrollable: difficulty and luck. You know those days when you forget to bring your umbrella and it starts pouring down rain and you curse the gods/the universe/your dumb luck? You are externally attributing your sogginess, instead of holding yourself responsible for not checking the forecast :) The other two factors are internal and entirely controllable: ability and effort. In language learning, it turns out that the difference between external and internal attribution makes all the difference.
Let’s take two learners, we’ll call them…Everett and Ida. Everett and Ida are in a language class together, and they just had a pop quiz today. Everett did very poorly, and he tells himself (and potentially anyone willing to listen) things like “Just my luck, on the week I didn’t have time to study,” “If she’d quizzed us on nouns and not verbs I would have done a lot better,” and “This is the hardest language in the entire universe.” Ida on the other hand did pretty well, and she thinks to herself, “Good thing I took time to study extra this week!” and “I totally kicked these verbs’ butts.”
Obviously in this scenario it seems that Ida is the more successful learner, because she did better on the quiz. However, it turns out that Ida is more likely to be successful in the long run even if she bombed this quiz. Ida attributes both her successes and her failures to her own ability and effort; she exhibits what applied linguists call self-efficacy. She understands that she has no one to credit, or to blame, for her accomplishments at this task other than herself. Everett, on the other hand, even if he does very well on this one quiz, is likely to chalk it up to good luck or easy questions. If everything is up to forces beyond his control, what reason does he have to put in any effort?
So this is what attribution theory tells us: in order to be successful at language learning, you have to grasp that you and you alone control your results. Other factors come into play, of course, but we all negotiate with a thousand such factors every day of our lives. You have to understand that you are capable, and you have to do the work.
 Brown, H.D. (2007) Principles of language learning and teaching. 5th ed. White Plains: Pearson. Pg 156
 I’m pretty sure I have said this, at one point or another, about every language I’ve ever studied.