I seem to waver between extremes - it's my undoing (7)
I believe the answer is:
nemesis
'my undoing' is the definition.
I can't judge whether this defines the answer.
'i seem to waver between extremes' is the wordplay.
'to waver' indicates an anagram (I've seen 'wavering' mean this).
'between' is an insertion indicator.
'extremes' becomes 'ns' (I am not sure about this - if you are sure you should believe this answer much more).
'i'+'seem'='iseem'
'iseem' is an anagram of 'emesi'.
'emesi' put into 'ns' is 'NEMESIS'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for nemesis that I've seen before include "Retribution personified" , "(Fate's) retribution" , "Archenemy" , "Goddess of retribution in Greek mythology" , "Vengeful justice" .)