Bitterly regretting (5)
I believe the answer is:
ruing
'bitterly regretting' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ruing that I've seen before include "Feeling sorry for" , "Feeling bad" , "full of remorse?" , "Bitterly regretting" , "Regretful" .)