The coating on ur tongue indicates the reason for ur bad Breath . Our tongue has multiple folds and numerous amount of thread like projections which are responsible for ur taste sensations in normal terms ur taste buds . Due to these multiple folds and projections tongue is a potent source of clogging debris and micro organism in them . May be u had nt been cleaning ur tongue ever since . As ur suggesting it is evident u might be having heavy debris and microorganism on ur tongue due to which it is discoloured and giving out foul smell . Start cleaning ur tongue with a tongue cleaner to remove the debris , and medicated mouth wash to eliminate the microorganism. U can also start chewing gums at day time temporarily to increase ur salivary flow which mechanically cleans ur tongue and teeth . Ur cavities can be cleaned and filled by a filling . If u have food lodgement in ur cavity even that can leave u with bad breath .