(1) Background: Grit is often celebrated as a predictor of youth success in sport, but little is known about how it develops over time through social and relational contexts. This study explores how grit forms among youth powerlifters through two developmental trajectories: an externally driven pathway shaped by structured adversity (Outside-In) and an internally driven pathway fuelled by self-motivation (Inside-Out). (2) Methods: This analytical autoethnographic study draws on multi-year coaching journals, field notes, and reflective narratives involving four Malaysian youth athletes. Through thematic coding and narrative synthesis, key developmental patterns were identified, and a dual-pathway conceptual model was constructed. (3) Results: The findings revealed that youth from lower-autonomy backgrounds often developed grit through coach-led discipline and adversity (Outside-In), while others showed early self-regulation and purpose-driven persistence (Inside-Out). Both pathways could converge toward internalised grit. The coach–athlete relationship was central in mediating this growth. Some youths later turned outward to uplift others, indicating broader developmental impacts. (4) Conclusions: Grit in youth sport is a socially embedded process. The Dual Pathway Model offers a framework for coaches and educators to cultivate perseverance in diverse youth through both relational support and autonomy development.