<p>Classical musicians’ individual practice time is central to their musical life and development, yet little research has examined the cognitive and affective processes that influence its effectiveness. This study investigates boredom, mind-wandering and flow during classical music practice, both on the trait and state level (i.e., during practice in general as well as during a specific session) as experiences associated with practice effectiveness. Classical musicians (<i>N</i> = 105) completed two 20-minute practice sessions and filled out questionnaires beforehand, between sessions, and after the second session. Boredom during practice emerged as a negative correlate for all performance outcomes: Trait boredom during practice was negatively associated with practice satisfaction, while state boredom was negatively related to both efficacy and creativity in a specific practice session. No significant associations were found between trait or state mind-wandering during practice and any outcome variables. Flow during practice, on the other hand, was positively associated with both efficacy and creativity. These findings suggest that while boredom is robustly linked to poorer outcomes and flow to better outcomes in classical music practice, the same is not true for mind-wandering. A possible explanation may lie in heterogeneous nature of mind-wandering and the relative freedom of practice. Given that boredom was consistently associated with less effective, less creative practice and lower practice satisfaction, whereas flow states were linked to more effective practice and higher creativity, future experimental or intervention-based studies should be developed to deepen our understanding of the mechanisms by which cognitive and affective states influence musicians’ practice.</p>