Choosing a field of study (study major) is challenging for prospective students. However, little research has examined factors measured prior to enrollment to predict motivation and well-being in a specific study major. Based on literature on affective forecasting and person-environment fit, prospective students' well-being forecast could be such a factor. However, affective forecasts are often biased by individuals' inaccurate theories about what makes them happy and their misconstrual of future situations. Thus, we hypothesize that subjective and objective interest-major fit forecasts improve predictions as these factors are based on a well-founded theory (person-environment fit theory) and objective interest-major fit forecasts are additionally based on a more accurate construal of the future situation (expert estimates of a study major). We tested these hypotheses in a longitudinal field study. Over 2 years, more than 4000 prospective students were asked for their well-being forecast and subjective interest-major fit forecast before using an online-self-assessment to assess their objective interest-major fit forecast. Of these prospective students, 234 subsequently entered the psychology major and took part in a survey about their motivation and well-being in their study major. As hypothesized, higher well-being forecasts predicted higher motivation, more positive affect, and higher satisfaction in the respective major. Beyond that, higher subjective interest-major fit forecasts predicted higher motivation, less negative affect, and higher satisfaction, while objective interest-major fit forecasts incrementally predicted higher motivation, more positive affect, and higher satisfaction. We discuss theoretical implications for affective forecasting and person-environment fit theory and practical implications for study orientation and guidance.