Peaches, maybe it is necessary for you to now work with these parts directly before you can progress further. Even though you internalised these parts from aspects of your parents, these parts are still 'you', and they have served a very useful purpose in your development.
It may not feel like it, but these parts likely served a protective role for you as a child. By taking on what your parents would say say or do, they preempted their reactions and protected you from further external abuse / derision / whatever. These parts may seem awful and abusive, but their original motviation may well have been to protect and not harm. Like all dissociated parts their jobs can cause trouble later in adulthood, but the good news is... they can be healed and integrated, just like the rest of you.
Maybe it's just time to find a way to work *with* them instead of in spite of them?