This study investigates how learners' English proficiency and beliefs about language learning change in short-term EFL and ESL programs. Using a mixed-methods approach, we examined TOEIC scores, language learning beliefs, and learning diaries from 41 students in an EFL program and 20 students in an ESL program. The results showed a statistically significant improvement in TOEIC scores for both groups. However, there were differences in the extent of improvement in Listening Comprehension and Reading Comprehension, depending on the learning environment. Additionally, changes in language learning beliefs varied, influenced by both the learning environment and the level of improvement in English proficiency. Notably, the EFL group and those who made greater improvements in English proficiency experienced significant increases in self-efficacy and confidence. Analysis of thelearning diaries revealed that learners faced challenges that prompted the development of learning strategies and motivation. These findings emphasize the fluid nature of language learning beliefs, which are influenced by learning environments and experiences. They suggest that positive shifts in learners' beliefs can improve learning outcomes, even in a short period of time.