from reading your post i took overbent to mean too much inside bend , however i think iloveshearer has taken it as behind the bit
from reding the reply , so not sure now , anyway...
for too much inside bend you need more outside aids , leg as well a rein , if the horse is still giving too much bend & most likely also leaning on your inside rein i would try a little outside flexion before asking the horse for the correct bend again , you might need to be really quite firm about this