Brief Summary: Fear of cancer recurrence (FCR) is a highly prevalent, disruptive, and under-treated problem for breast cancer survivors. This randomized controlled trial will test the efficacy of group-based Acceptance and Commitment Therapy compared to Cognitive Behavioral Therapy and enhanced usual care for breast cancer survivors suffering from FCR while examining its cost-effectiveness and the mechanisms by which the intervention may work. Study findings will guide the future care of breast cancer survivors with FCR.

Detailed Description: The primary objective of this 3-arm randomized control trial (RCT) is to build on the investigators' pilot work by testing the impact of Acceptance Commitment Therapy (ACT) on FCR. The investigators will randomly assign up to 375 early-stage Breast Cancer Survivors (BCS; for at least 300 study completers after attrition) who have finished primary cancer treatment and who report clinically significant FCR to: (1) group-based ACT (6 weekly 1.5-hour videoconference sessions), (2) group-based Cognitive Behavioral Therapy (CBT; 6 weekly 1.5-hour videoconference sessions), or (3) Enhanced Usual Care (EUC; a single 90-minute videoconference coaching session with self-administered readings). Outcomes will be assessed at baseline and at 2, 6, and 12 months; additionally, potential theory-driven mediators of the ACT intervention's effects on key outcomes will be analyzed at these time points and at intervention midpoint. Cost-effectiveness of each intervention will be assessed. Specific Aims are to: (1) test the efficacy of group-based ACT compared to CBT and EUC on FCR (primary outcome) and anxiety, depressive symptoms, post-traumatic stress, avoidant coping, fatigue, sleep disturbance, and quality of life (secondary outcomes) in BCS with clinical FCR; (2) to examine changes in psychological flexibility as a mediator of ACT's effect on FCR; and (3) to perform comparative assessments of ACT, CBT, and EUC to determine the cost-effective intervention.