Sentence:  These two truths coexisted in my mind without overlapping until I bit into a piece of crumb cake so texturally perfect (soft sliver of cake topped by a deep layer of grape-size crumbs), yet so <WSD>toothachingly</WSD> sweet that the only antidote was sucking on the lemon in my seltzer. 
Meaning of the word marked with <WSD> tags:  So as to cause toothache.