a 15mm stone is not too big I´ve passed stones as large as 40mm with no difficulty and no pain at all.
many of my Gallstones were over 30mm.I´ve push thousands of stones. You might be surprised as you may have stones which are larger than 15mm in your liver and they will be eliminated with a flush.
Go for a flush asap and i am sure you´ll be able to push this gallstone.