Psychological flexibility (PF) is defined as one’s ability to pursue valued activities despite distress. PF is a critical process of change in evidence-based treatments, and is associated with psychosocial health and functioning. Although PF is considered context-dependent, previous research often measures PF as a static construct, often by administering the Acceptance and Action Questionnaire, which may not fully capture the construct of PF and limits understanding of how PF may change over time. One approach for measuring individual dynamics over time is ecological momentary assessment (EMA), which has been applied to numerous psychological constructs, including PF recently. This study investigated the dynamic relationship between PF, affect, and stress in a clinical sample of 39 individuals. Six items from the Process Based Assessment Tool were used to measure PF in terms of experiential avoidance and values-promoting processes. Participants completed a two-week EMA phase which included answering daily self-report items, and collecting smartphone and wearable technology data on screen time, steps, sleep quality, distance traveled, and activity. I hypothesized that PF would vary within and across time and context to predict affect and stress and expected that indicators of psychosocial health and measures of psychological processes would influence PF, affect, and stress. Results revealed significant associations such that flexibility was generally related to higher positive affect, lower negative affect, and lower stress. Some PF-items were associated with better day-quality ratings. PF interacted with context (conflict or valued action) and type of situation, with greater PF generally associated with valued-actions. Measures of psychological and attentional processed differentially interacted with PF to predict affect and stress. Step count interacted with PF in several models. Screen time was associated with affect and stress at a given timepoint. Heart-rate variability was differentially related to stress, affect, and PF within and across time. Activity, GPS, and sleep quality data were not significant. Overall, this study supports evidence that PF is highly idiographic and related to indicators of psychosocial wellbeing over time, generally supporting my hypotheses.