Second language (L2) motivation research is in the midst of a theoretical reckoning. The L2 Motivational Self System, the leading theory of the field, has drawn recent critiques, including calls for a moratorium on its use (Al-Hoorie et al., 2024a). These critiques have prompted scholars to seek theories with stronger empirical grounding and greater parsimony (McClelland & Larson-Hall, 2025; Oga-Baldwin, 2024). Self-determination theory (SDT) has emerged as one leading candidate, supported by a growing body of second language acquisition (SLA) research (e.g., Al-Hoorie et al., 2025; Noels, 2023), yet its account of motivational sustainability leaves open why need-supportive conditions do not always produce lasting engagement. This talk introduces identity-based motivation (IBM) theory (Oyserman, 2015) to SLA, focusing on why the same experience of difficulty sustains one learner and derails another. IBM addresses this through the construct of difficulty interpretation. When a learner's L2 identity is salient, experienced difficulty is read as importance, confirming that the struggle belongs to becoming a competent language user. When that identity is not salient, the same difficulty is read as impossibility, and disengagement follows (Strietholt, 2026). By situating difficulty interpretation within a synthesis of IBM and SDT, the talk specifies the mechanism SDT leaves unaddressed and offers instructors a concrete basis for reframing difficulty in the classroom rather than minimizing it