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.
彼は内心忸怩たる思いで過去の失敗を振り返った。
He looked back on his past failures with deep inward shame.
内心忸怩として、その場に立ち尽くした。
Overcome with private mortification, I stood there frozen.
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.