Loneliness is a global public health concern with documented associations with morbidity and mortality. Recent studies indicate that self-control and loneliness are reciprocally associated. Self-control refers to the capacity to inhibit impulses and align behavior with long-term relational goals. Lower self-control can degrade relationship quality through norm violations and social withdrawal, thereby increasing perceived exclusion and loneliness. In the opposite direction, loneliness can deplete cognitive and motivational resources and bias social information processing toward threat appraisals, which in turn compromises subsequent self-control. However, evidence at the day-to-day, within-person level remains limited in midlife and older adults. We analyzed a 7-day daily diary study of middle-aged and older adults in Korea (N = 630; age 40-71 years; Mage = 58.28; 56.5% men). Each evening, participants reported self-control (K-BSCS), loneliness (3-item UCLA Loneliness Scale), and perceived social exclusion (Ostracism Short Scale) using validated instruments adapted for daily assessment. A random-intercept cross-lagged panel model (RI-CLPM) partitioned within-person temporal dynamics from stable between-person differences, adjusting for demographic and health covariates. At the between-person level, lower average self-control was associated with greater loneliness and social exclusion. Within persons, lower-than-usual self-control prospectively predicted higher next-day loneliness (e.g., day 5 to 6: b = -.266, p = .006), and higher-than-usual loneliness prospectively predicted lower next-day self-control (day 6 to 7: b = -.097, p = .017), evidencing reciprocal coupling. Perceived social exclusion was related to both constructs but showed no evidence of mediating their day-to-day linkage. Findings indicate a reinforcing cycle connecting self-regulatory capacity and social experience in later life; accordingly, interventions that enhance self-regulation or reduce loneliness may yield spillover benefits for the other. Methodologically, results underscore the value of intensive longitudinal designs and RI-CLPM for isolating within-person mechanisms relevant to prevention and healthy aging.