Our anger and annoyance are more detrimental to us than the things themselves which anger or annoy us.

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations, Book 11, #15 [tr. Staniforth (1964)]
    (Source)

Alternate translations:

How many things may and do oftentimes follow upon such fits of anger and grief; far more grievous in themselves, than those very things which we are so grieved or angry for.
[tr. Casaubon (1634)]

Consider that our anger and impatience often proves much more mischievous than the provocation could possibly have done.
[tr. Collier (1701), #18]

Consider how much more pain is brought on us by the anger and vexation caused by such acts than by the acts themselves, at which we are angry and vexed.
[tr. Long (1862)]

Consider that our anger and impatience often prove much more mischievous than the things about which we are angry or impatient.
[tr. Zimmern (1887)]

How much more grievous are what fits of anger and the consequent sorrows bring than the actual things are which produce in us those angry fits and sorrows.
[tr. Farquharson (1944)]

Anger and the sorrow it produces are far more harmful than the things that make us angry.
[tr. Needleman/Piazza (2008)]