Hmm...have a similar kind of experience. One way I explain it is my mind doesn't understand the existence of anything other than itself. An imagery would be a rampart surrounding myself that doesn't permit access to objective reality.
It's a real toughie and can easily lead to being ungrounded, escapism, fantasy and the use of defence mechanisms - because it really does seem the mind is determined to stay in this way.
I have never been diagnosed with anything, BPD included. However I find I fit the quiet BPD bill more so because rarely in life do I rage or act out. I 'act in', swinging from pity to indulgence and seldom recognizing compassion given from myself or others because of the vast barriers to perceiving reality as is living in one's own head creates.
It's a difficult one. I am working on this too. I think genuine involvement in community and groups in a consistent manner can aid, provided the group has aims that are beneficial or work on such aspects. An example would be DBT; although I have never tried it, I am enthusiastic to.
Something like this is quite a process to undertake.
Not sure of how. If anyone else has any suggestions, please share! It's about time this mystery gets cracked