That past event - painful to even think about - I don't want to bring up again.
Tips
grammar
Classical-flavored 4-character pattern: 思 ('to think of') + 之 (classical pronoun 'it') + 心痛 ('heart pains'). Literally 'thinking-of-it heart-pains'. The 之 here is the same classical object pronoun as in 知之为知之 ('to know it is to know it').
register
Literary, used in essays, eulogies, and sentimental writing rather than everyday speech. Sometimes paired with 念之伤怀 ('to be saddened by remembering it') for parallelism.