I'm sorry you feel vulnerable and want to crawl under a chair, although I can understand it. I like what franciemarnie says, about actually crawling under a chair... or maybe a table, realistically? I used to have to get under a table when I lived abroad and there were earthquakes (which seriously scared me), and it can be quite comforting.
Sternum is the breastbone, right? When I had craniosacral therapy, the practitioner was also fully trained as an osteopath and one session he felt that my breastbone needed adjusting, which apparently meant him pressing down on it incredibly hard and me trying to remind myself that he was a trained osteopath and knew what he was doing.
The sensation of him doing that gave me two very strong feelings about this area of the body. One was that it was very protective, like it was a very thick bone. Almost like the skull - a kind of massive protection. So if someone presses down on it, the bone takes the pressure. It's protecting what's underneath.
The second feeling was that what's underneath is very, very vulnerable, being the lungs and heart. There's something about the most fundamental survival here. I remember my first aid training and what we were taught to put In order of priority: breathing, bleeding, then broken bones. So the lungs are above everything, the heart is next, and everything else is below that.
I felt afraid when he was pressing down so hard, but I could also feel how much this bone was between him and my lungs and heart, protecting them. I think I felt those things equally at the same time.
In terms of feelings, I'd say that for me what I'd associate this with is closest to shock. In an objective sense that could be any kind of shock - physical trauma, emotional trauma, any big impact on the system. In a less subjective and PTSD sense, personally I would relate that feeling to things becoming more real. A layer of protection falling away. The immediate impact on breathing/existing and feeling.
So, from my viewpoint, I'd probably see this as progress and moving forward. I'd also see it with a great deal of sympathy that progress and moving forward are usually very, very far from easy. A lot of adjustement can be needed.
I just bought Focusing by Eugene Gendlin. He gives a method to find out what's beneath that ambiguous sense in a gentle way but I haven't finished the book or tried it yet.
My craniosacral therapist/osteopath recommended that book to me. I'd like to completely advocate for it and at the same time say - be careful. Tuning into your body could suddenly blow open all sorts of things. But tuning into your body - carefully, in a contained, protected way, and with a lot of safety around that - can be astonishing.