こっそり (translations)→ creep discreetly furtively quietly sott-voce stealthily surreptitiously unperceived unseen
ひそかに (translations)→ mentally private secretly silently softly unadvertised
ひとりで (translations)→ alone full-time helpless oneself single-handed unilaterally
個人的に (translations)→ personal personally
内緒で (translations)→ semi-privately
内心 (translations)→ actually conscience enjoy enormously inside internal inward inwardly secret sneaking truth within
