Background: The psychological flexibility model states that activity patterns are not deemed to be intrinsically functional or dysfunctional; it is considered that underlying factors, such as personal goals and contextual factors, are what will determine their effects on disability. Pain catastrophizing has frequently been associated with several important pain-related outcomes. Despite its recent conceptualization within affective–motivational approaches, its moderating role between activity patterns and dysfunction has not been analyzed. Methods: This study analyzes the moderating role of pain catastrophizing and its dimensions (Pain Catastrophizing Scale) between activity patterns (Activity Patterns Scale) and disease impact (Fibromyalgia Impact Questionnaire—Revised) in 491 women with fibromyalgia. Results: Activity avoidance (p < 0.001), excessive persistence (p < 0.001) and pacing (p < 0.01) patterns were positively associated with fibromyalgia impact. Helplessness shows a moderating role between pain avoidance (B = 0.100, t =2.30, p = 0.021, [0.01, 0.18]), excessive persistence (B = −0.09, t = −2.24, p = 0.02, [−0.18, −0.01]), pain persistence (B = −0.10, t = −2.04, p = 0.04, [−0.19, −0.004]) and functioning. Conclusion: Helplessness (within pain catastrophizing) is a relevant variable within psychological flexibility models applied to activity patterns. Specifically, pain avoidance is especially dysfunctional in patients with high helplessness. To improve excessive persistence and pain persistence, it is necessary to reduce helplessness before regulating activity patterns.