A modest one, to bear me back again."
This would seem to mean that, notwithstanding her boldness, he would still continue to think her modest. It would be derogatory to Helena to read sum for 'one.'
"When miracles have by the greatest been denied."
Johnson saw rightly that a line had been lost after this.
"Where hope is coldest and despair most shifts."
For 'shifts' Pope read sits, Theobald fits, which I think is right; fits occurs in the sense of suits in
"The humble salve which wounded bosoms fits."
Son. cxx.