It is very difficult to filter other people's opinion of ourselves, especially when we are in a vulnerable state.
It's not that you aren't being yourself when you don't react or hold yourself back, it's that you are shielding/masking your emotions so that they cannot be used against you. It is a natural defense mechanism, but it does take up more energy than you can afford to spend when you are feeling so trapped already.
When it is a family member or some one you care about that is being manipulative or harsh to you, it cuts you even deeper.
When my family yells at me, I am overwhelmed with pain and a sense of betrayal. I have a tendancy to shut down and lock them out. It is not the best way to handle things, but it is the only way that I can feel safe until I have the energy to handle the emotions.
I wish I had a direct answer or fomula to give you, but unfortunately, it is an inner balance that only you can find what works for you.

You might not see anyone in the room with you, but you are not alone in your struggles. I hope you can find some comfort in that.