Halliday & Matthiessen (2014: 274, 275, 275n):
There is overlap here between ‘mental’ and ‘relational’ clauses, and some clauses such as I was scared, could be interpreted either way. There are four main indicators: (1) submodification; (2) marked phase; (3) tense; and (4) clause structure. …
But these four criteria do not always coincide, and not every instance can be clearly assigned to one category or the other.¹
¹ Thus we find clauses where the participial form of the mental process verb is submodified but where the Phenomenon is also present in the form it takes in a ‘receptive’ clause; for example: she was very intrigued by alternative ideas; I was very depressed by some feedback I was getting.