My costochondritis from a many years ago is causing agonizing pain. What should I do?

Rest / meds. If we are sure this is costochondritis, treatment is judicious rest, ice, heat, non steroidal meds and tincture of time. If persistent and pain localized may benefit from injection or topical meds. Physical therapy, massage is also possible. Systemic pain meds would be last option in my mind.
Depends. Nsaids such as Ibuprofen are the mainstay of treatment for this. However, if you have any doubt, see your doctor for a full work-up.