Ask your doc if it might be a bone spur hitting the nerve, and ask whether a CT scan might show that better than your previous MRI and myelogram. Ask the doc whether oral steroiRAB for a week or two might do a better job of attacking the inflammation. Also ask your doc if some stronger pain meRAB might be in order, as least for a brief period, to give you a break. Ask if perhaps some pain relief might give you a chance for some healing.
I'm so sorry you have this sciatica which is the worst pain there is.