。
I regret telling the truth.
Literal
True [possessive-の] thing [object-を] said thing [object-を] regretting.
後悔している ('am regretting') marks an ongoing state of regret. 本当の事を言った事 — two uses of 事 (こと): the first nominalizes 'the truth' and the second nominalizes 'having said it.' The stacking is grammatically correct but slightly heavy.