D. laureola is a rare wild plant in Britain, and is found mainly in chalky woodlands. I've found it a couple of times in the Chilterns, in places looking so dry in the summer you wonder how it survives. My father has one in his very dry, chalky, heavily treed garden and it grows fine without any attention or watering. Maybe you are overwatering it, and maybe it would like a bit of lime. Maybe it would like to be put in the ground and suffer some benign neglect. A few leaves going yellow and falling off at this time of year is normal. The rather similar looking D. odora does the same. D. odora is a much more attractive plant in flower to my taste, but each to his own.