The goal of this experimental study is to learn if the Booster intervention can be used as an early intervention for persistent fatigue. The aim of the intervention is to prevent more fatigue-related disabilities in young people with chronic illnesses like inflammatory bowel disease, childhood cancer, and juvenile idiopathic arthritis by improving self-confidence in managing fatigue (fatigue-related self-efficacy or FSE). The main questions this study aims to answer are: * What is the effect of the Booster intervention on fatigue-related self-efficacy (FSE)? * What is the effect of the Booster intervention on fatigue levels, school participation, life satisfaction, and perceived health? * When does improvement in study outcomes happen relative to the Booster intervention? * What participant characteristics predict change in study outcomes? Participants will follow the Booster intervention. Booster is a personalised, blended care intervention designed to help young people understand how their thoughts, feelings, and activities impact their fatigue using a smartphone app. The Booster app uses experience sampling methodology (ESM) to track these fluctuations. Based on insights obtained from these data, the participant and their healthcare provider can set personal lifestyle goals, such as increasing physical activity and reducing daytime naps. An earlier version of this intervention, called PROfeel, was effective in reducing severe fatigue. The investigators have now improved the app to also help with setting and achieving personal goals and monitoring daily progress. The investigators will measure the effect of Booster through daily questionnaires during two phases: Phase A (baseline, before the goal setting between patient and healthcare provider) and Phase B (intervention, starting in the lifestyle change period). The duration of Phase A will be randomised for each participant.
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Fatigue-related self-efficacy
Timeframe: Daily before and during the intervention, for approximately 130 days