Rumination, a cycle of repetitive negative thinking, is a transdiagnostic factor driving mental health struggles across populations. In this PhD thesis, I explored its cognitive, affective, and neural mechanisms in early adulthood and work, alongside evaluating personalised neurostimulation as a potential intervention, across five studies. In Study 1, I examined the lived experiences of depressive rumination in early adulthood through reflective thematic analysis (N=20), generating five themes: unresolved trauma, contingent self-worth, stigma-induced isolation, a relentless pursuit of inner peace, and wider wellbeing repercussions. In Study 2, I conducted a quantitative examination of depressive rumination, work-related rumination, and inner speech in a wider age range (N=203). Brooding, work-related affective rumination, and evaluative/critical inner speech predicted increased depression, anxiety, stress, and cognitive failures. Conversely, positive/regulatory inner speech was associated with decreased depression and improved inhibition and shifting abilities, with the latter benefit emerging particularly under low maladaptive rumination. In Study 3, I applied network analysis to explore rumination, perfectionism, and stress in teachers (N=1344), identifying depressive brooding and work-related affective rumination, as well as striving for excellence, as key intervention targets linking stressors to wellbeing outcomes. In Study 4, I developed a personalised Bayesian algorithm to optimise transcranial alternating current stimulation (tACS) parameters for teachers with high work-related affective rumination across 399 sessions (N=67). High amplitude and low frequency tACS were the most effective in reducing rumination. In Study 5, a follow-up double-blind, within-participant experiment, I found that both personalised and sham stimulation reduced rumination, with no significant differences between them (N=38). However, reductions in rumination were associated with reductions in sleepiness at higher current amplitudes. Overall, my thesis provides novel evidence of rumination’s complex, multifaceted impacts on cognition, emotion, and physiology, and explores the potential of personalised interventions to mitigate its adverse wellbeing outcomes in personal and occupational contexts.