Your neighbor owns the disputed property through adverse possession,

This is usually a matter that will have to be resolved in court unless you and your new neighbor come to an agreement. See if you have any surveys of your property. You can probably go to your county clerk's office and obtain copies that were part of the various mortgage and deed filings going back in time. It should also be noted on your title policy whether there are any disputed boundaries that contradict the strict language of the deed.