Inclusion Criteria:  Within 5 years of completing curative treatment (surgery, chemotherapy, and radiation) for stage 0-III breast cancer <60 mins/week moderate-vigorous physical activity with no major changes for the past 6-months (objectively verified during the baseline assessment) Own an Android or iPhone smartphone and willing to use the smartphone to complete app-based surveys during assessment periods Willing to wear the ActiGraph monitor during assessment periods Access to internet to complete REDCap survey assessments  Exclusion Criteria:  Non-English speaking/not able to read English Evidence of major contraindications for exercise (informed by the 2020 Physical Activity Readiness-Questionnaire (PAR-Q)+) Currently pregnant History of severe mental illness or currently taking mood stabilizing medications (antipsychotics, anticonvulsants, or lithium) Evidence of moderate-severe depressive symptoms (indicated by a score ≥10 on Patient Health Questionnaire-8) Evidence of moderate-severe cognitive impairment (indicated by a score < 3 on a 6-item cognitive screener) Use of mood-altering substances (alcohol, cannabis, nicotine, opiates, etc.) daily or nearly every day