、。
She regretted and reflected on her serious mistake.
Literal
She [topic-は] serious mistake [object-を] regretted, reflected-on.
Two verbs are listed with the reading comma: 後悔した (regretted) and 反省した (reflected on / felt remorse). 後悔 is emotional regret — wishing you hadn't done it — while 反省 is more constructive self-reflection, examining what went wrong. Japanese often pairs these to show both the emotional and rational responses to a mistake. 重大な過ち uses 過ち (ayamachi), a slightly literary word for a moral error or transgression, heavier than ミス or 間違い.