The part in bold appears in lines 558–559 of the Storr edition:
τέως δὲ κούφοις πνεύμασιν βόσκου, νέαν
ψυχὴν ἀτάλλων, μητρὶ τῇδε χαρμονήν.
Meanwhile let light airs feed thee; cherish thou
Thy tender ...
TL;DR: The assignment of lines to speakers comes down to the judgment of the editor or translator, based on their sense of what is dramatically most appropriate in a given case, and whether their goal ...
The Greek text is:
μέγαν ὄκνον ἔχω καὶ πεφόβημαι
πτηνῆς ὡς ὄμμα πελείας.
Sophocles. Aias, lines 139–140. Perseus Digital Library.
ἔχω = I bear/carry
μέγαν ὄκνον = great fear
καὶ = and
πεφόβημαι = I ...