My Theory:

Regina was so depressed and fled from the kings castle. She found Rumple (somehow) and begged him for help. He said that he could make her 'powerful' and afraid of 'nothing.'

But.....she turned 'bad' (Same as Rumple needed help and HE turned 'bad' also.) Regina could NOT be cured because her 'true love' is dead...So she remained 'cursed'.

When and IF Rumple takes her powers AWAY...will she turn 'good' again???!