How can I deal with foot pain caused by flat foot?

Address the problem. Flatfeet can cause any number of symptoms and problems. You need to address what we call the hyper pronation, this is done with a pair of custom orthotics. They are more than just arch supports, they would change how the patient stands and functions. Consult your local area podiatrist for these devices.

Related Questions

How to deal with foot pain caused by flat foot that was high arch?

Orthotics. Or arch supports are very helpful or this condition known as a pronating foot.
Not good. The most common cause of a high arched foot becoming flat is rupture/tear of the posterior tibial tendon. See a podiatrist and get this checked out.
Have it evaluated. Have your feet evaluated by a professional and get the appropriate treatment. You may benefit with use of custom foot orthoces.