This paper reports on the process of developing an original framework for conceptualising and measuring language learner well-being within the context of heritage language (HL) learning. Drawing on a quantitative validation study with 545 young Chinese heritage language (CHL) learners, aged 7-18, in the UK, this paper presents an empirically validated model alongside a self-report instrument designed to measure learner well-being. A comprehensive development and validation process yielded a 24-item "Language Learner Well-being Scale," exhibiting robust psychometric properties, including face, content, construct validity, and internal reliability. Results from exploratory and confirmatory factor analyses uncovered a four-factor model of language learner well-being -- enjoyment, connectedness, meaning, and competence -- which represent the key pillars of flourishing among CHL learners. This multi-dimensional model provides potential pathways through which flourishing can be actively promoted in HL learning, with implications that are applicable to wider second/foreign language learning contexts. By exploring what constitutes well-being and developing an effective measure, the study also sets the stage for meaningful interventions in the future to optimise students' language learning experience.