taru-adjective, adverb taking the 'to' particle
ashamed deep down; inwardly mortified; blushing with shame at a memory
Literary, somewhat old-fashioned expression for a deep, private feeling of shame or embarrassment, often when recalling a past action. Used with たる or と as a たる adjective or adverb.
He looked back on his past failures with deep inward shame.
Overcome with private mortification, I stood there frozen.
恥ずかしい is the common everyday word for 'embarrassed' or 'ashamed'. 内心忸怩 is a much rarer, literary expression for a deep-seated, private shame.
忸怩 alone means 'ashamed' and is also literary/rare. 内心忸怩 combines it with 内心 (inwardly) to emphasize the internal nature of the shame.
Standard kanji form; 忸怩 is a rare kanji compound meaning 'ashamed'.
Mixed kanji-kana spelling; used when the second part is written in kana for readability.
Compound of 内心 (naishin, 'inwardly; in one's heart') and 忸怩 (jikuji, 'ashamed; mortified'). 忸怩 is a rare Sino-Japanese word, and the whole phrase is a fixed literary expression.