This article argues that standard models of person-centred care (PCC) and shared decision making (SDM) rely on simplistic, often unrealistic assumptions of patient capacities that entail that PCC/SDM might have detrimental effects in many applications. We suggest a complementary PCC/SDM approach to ensure that patients are able to execute rational decisions taken jointly with care professionals when performing self-care. Illustrated by concrete examples from a study of adolescent diabetes care, we suggest a combination of moral and psychological considerations to support the claim that standard PCC/SDM threatens to systematically undermine its own goals. This threat is due to a tension between the ethical requirements of SDM in ideal circumstances and more long-term needs actualized by the context of self-care handled by patients with limited capacities for taking responsibility and adhere to their own rational decisions. To improve this situation, we suggest a counseling, self-care, adherence approach to PCC/SDM, where more attention is given to how treatment goals are internalized by patients, how patients perceive choice situations, and what emotional feedback patients are given. This focus may involve less of a concentration on autonomous and rational clinical decision making otherwise stressed in standard PCC/SDM advocacy.