Adolescence is a crucial time of identity formation, scholastic and social demands, future concerns, and relationship changes. Adolescents are especially susceptible to psychological issues like pessimism, which can hinder their progress and well-being. Cognitive flexibility may help adolescents adjust to these challenges and improve subjective well-being. Pessimism, cognitive flexibility, and subjective well-being have been investigated cross-sectionally, but their longitudinal association has not. This study examines cognitive flexibility's mediation function in the longitudinal association between pessimism and subjective well-being in teenagers, taking into account their well-being development and inadequacies. This study looked at how cognitive flexibility affects the link between pessimism and subjective well-being. To address the limitations of cross-sectional mediation analysis, the current study employed an autoregressive cross-lagged panel model within a half-longitudinal framework, which allows for a more accurate estimation of directional and temporal relationships among the variables. This model used two 3-month-apart data sets. Cognitive flexibility was found to mediate the relationship between pessimism and subjective well-being (X[superscript 2] (3, N = 232) = 11.68, p < 0.001). These findings indicate that cognitive flexibility plays a significant mediating role in weakening the negative impact of pessimism on adolescents' subjective well-being, highlighting its importance as a protective cognitive factor during this critical developmental period.