The current study was conducted to investigate the relationship between math anxiety and math achievement, highlighting differences in motivation based on age, and the mediating effects of task persistence as well as aspects of the expectancy-value theory. The students in the study (grade five N = 335, grade eight N = 251) were from Estonian schools and completed questionnaires about achievement motivation as well as math tasks during the 2021/2022 school year. Results showed that students in grade eight report lower scores in motivation when compared to students in grade five. Additionally, task persistence mediated the relationship between math anxiety and math achievement in grade five but not in grade eight. Self-efficacy, interest value, and effort cost mediated the relationship between math anxiety and math achievement in both grades five and eight, while attainment value and utility value did not. This indicates that the possible positive cognitive and affective effects aid in relieving, while effort cost may increase the negative effect of anxiety on math achievement.