I've got two questions. First of all, what does she had felt secure in his comprehending pity mean. Second, what does she was flung back on herself mean.
All her old resentments and rebellions flamed up, confusedly mingled with the
yearning roused by Harney's nearness. Only a few hours earlier she
had felt secure in his comprehending pity; now she was flung back on
herself, doubly alone after that moment of communion.
Please!
.
secure in his comprehending pity => sure that he would understand and pity her
(another possible meaning would be 'safe in the knowledge that he understood and pitied her', but I don't think the context would allow that)
flung back on herself => violently returned to a state of having to fend for herself/rely on no one but herself
b