You might consider having the tax assessor re-evaluate your taxes based on the lower value. That would save you some property tax. There's not much you can do about the mortgage payments. My son has successfully worked out some principal reductions for a few homeowners. The lenders preferred taking a small bath to a complete wipe out. The borrowers had a good payment record. It just a hell of a lot of work to shepherd the negotiations through all the necessary parties.
In a lot of places they just laugh at you and tell you to wait until the next reappraisal.