tasya vriiDena mahataa
pramodo hRdi n' abhavat
n' aavatasthe vrate manaH.
Because of his great shame
The exuberance in his heart was no more.
His mind left downcast due to disenchantment,
He could not remain steadfast in his observance.
This verse is certainly about shame, and it also seems to be about grief.
The first thing this verse seems to tell us about shame is that it can't co-exist with exuberance -- in accordance with the pyschological principle that the human mind can't experience two opposing emotions at once.
Shame is a kind of suffering whose cause, as with all suffering, lies in faults. The pain of Nanda's present shame -- his consciousness that his motivation was wrong, and that his naive assumption about the permanent bliss of heaven was wrong -- is part of a process that will eventually lead Nanda to abandon the faults which are the cause of suffering.
Nanda's disenchantment might be a kind of bereavement. Nobody has to die for a bereavement to be suffered. Loss of a beautiful romantic dream also causes the body to enter the bereavement process, beginning with white-faced shock that sets the wheel of samsara in motion.
In 10.63 the Buddha has promised Nanda that if he is steadfast in his observance, i.e. if he adheres unerringly to the rules of discipline, he will enjoy union with the apsarases. It may be that the Buddha made this promise safe in the knowledge that Nanda would inevitably become disenchanted, and that whenever anybody becomes disenchanted, they cannot remain steadfast. Rather, as part of a hard-wired bereavement process, they are invariably shaken and stirred.
It may be that the kind of steadfastness described in 16.84 can only occur after disenchantment, never before.
The feeling of deep shame made exaltation leave his heart, and his mind, downcast for lack of exaltation, did not abide firm in his vow.
Because of his great shame, there was no pleasure in his heart, and through lack of pleasure his depressed mind could not persist in his stated intention.
tasya (genitive): of him, his
vriiDena (instrumental): because of shame
mahataa: f. greatness , mightiness
pramodaH (nominative, singular): excessive joy , delight , gladness
hRdi = locative of hRd: the heart
abhavat = past tense of bhuu: to be, to have
praamodyena = instrumental of praamodya: n. rapture , delight
vimukha: having the face averted , turned backwards ; turning away from (gen.) , disappointed , downcast
avatasthe = 3rd person singular of ava-√sthaa: to take one's stand , remain standing ; to stay , abide , stop at any place (loc.)
vrate = locative of vrata: will , command , law , ordinance , rule ;
obedience , service ; sphere of action , function , mode or , manner of life ; conduct , manner , usage , custom ; a religious vow or practice , any pious observance , meritorious act of devotion or austerity , solemn vow , rule , holy practice (as fasting , continence &c ; vrat/am- √car , " to observe a vow " , esp. " to practise chastity "); any vow or firm purpose
manaH (nominative, singular): mind