I wouldn't take the advice at the time; now there's no fixing it.
Tips
history
Traced to Chao Cuo's Han-era memorial 上书言兵事: '跌而不振，则悔之亡及也' - 'to stumble and not recover: the regret comes too late'. The modern form 追悔莫及 preserves that sense of irreversible regret.
memory
追 here means 'to chase after (retroactively)'. 追悔 = 'regret after the fact'. 莫及 = 'cannot reach / catch up'. You can chase your regret but never overtake what's already done.
