This sounds like what happens to me when I try to meditate and also in yoga. I was trying to explain it recently in my mindfulness class; I think people with CPTSD need a different approach. The problem is when you are hyper vigilant, highly armoured and defended anything that relaxes you feels like your guard is down and defences lowered, which can either provoke an immediate shock like response (eg in yoga during deep relaxation I often startle when I suddenly hear a sound or just become aware I have let my guard down for a second), or sometimes when it's not as deep a response you might feel a bit better for a while but gradually you start to feel a sense of unease and vulnerability as you have lowered your defences or moved out of your comfort zone. I am gradually finding ways to carefully pace deep relaxation and meditation with breath work and use of objects and aromatics to anchor or ground myself but there's a long way to go and some poses and practices I just avoid at the moment.

So while I would agree that meditation and yoga are probably good for you in the long run, as is mindfulness and relaxation, those of us with CPTSD need to think carefully about how these approaches need adapting and pacing to get around these barriers.