I think feeling it is what's really important here. Something that may help you, adapted in whatever way is meaningful to you, is a way to make these feelings more tangible. Perhaps your T would be willing to consider a similar exercise. My T led me through a visualization to help me find an image that could counter that sense of borderlessness.

We settled on the image of sea glass because as a child I would spend hours combing the beach searching for and collecting sea glass. In the visualization, we imagined the pieces of myself as bits of sea glass, and together, gathered them up. This wasn't hypnosis, but was done in a state of relaxation in which I provided the images, and he included himself in the action. So it was a co-created visual story.

The following week, he brought a glass bowl half full of sea glass to session. We repeated the visualization while I held the bowl on my lap. Then, I handed him the bowl, and he held it while I focused my attention on his holding it. This may have lasted @ 10 minutes, with his occasionally saying phrases to help me focus on the image/feeling of him holding the bowl, until I felt something shift inside me. Difficult to describe exactly what the shift was, but I experienced it as an inner sense of calm, safety--being held. We repeated this sequence many times over the next few months, and periodically whenever I felt particularly vulnerable. It was also important to me that he keep the bowl on his desk. So when I would feel fragmented outside of session, I could re-imagine his keeping the parts of my self safe. Eventually, I could summon up the feelings on my own in and out of session without the bowl.

The theory is related to the use of transitional objects--Linus' blanket--but more representative of my sense of fragmentation, with the goal of me being able to hold myself, rather than a representation of him for comfort.

Perhaps this is related in some way to Hankster's use of dolls in session?