innermost depths of one's heart
Refers to the deepest, most private feelings or thoughts; often used in emotional or introspective contexts.
His words resonated in the innermost depths of my heart.
Deep down in her heart, she still loved him.
Compound of 心 (こころ, 'heart, mind') + の (possessive particle) + 奥底 (おくそこ, 'deepest bottom, innermost depths'). The expression is transparent and follows standard Japanese syntax.