Halliday & Matthiessen (2014: 275):
Submodifiers like so, very, too go with nominal groups but not with verbal groups: we can say I was very afraid of it but not I very feared it; you’re not too keen on it but not you don’t too want it. All the words listed in Sensing as Attribute and as Process as ‘adjective/participle’, glad, sorry, worrying, frightening, etc., readily accept these submodifying items; hence a clause featuring be + worrying, frightening etc. is likely to be ‘relational’ rather than ‘mental’.