A low pressure headache could occur this far out from an epidural procedure, and may persist indefinitely in a minority of cases, unless a source for the leaky cerebrospinal fluid is found.
The sign looked for on MRI is enhancement of the meningeal lining of the brain. Unfortunately, the actual epidural procedure itself can cause the same finding, so even if it shows enhancement it is hard to seperate which is the cause.
A blood patch may help, also sometimes a saline infusion into the CSF space. Some data shows that an infusion of a solution of ACTH (a body hormone) benefits low pressure headache. Tests can be done to localize a CSF leak such as radionucleotide cisternography or a CT myelogram - ask your doctors about these tests. Persistent headache Usually does not cause permament damage - in older people if it is severe and persistent, subdural effusions can develop.
As your headache is now chronic, there may be other factors perpetuating the headache such as OTC medication overuse and neck/muscle spasm - these issues will need to be addressed also before your headaches become under control.
Good luck