Hi Happy Head!
I have a trauma therapist who specializes in mindfulness, and teaches me how to stay present and grounded as well.
What your T is describing is true - I noticed this in the beginning, and yet I'm practiced in spending long times in contemplative prayer without dissociating. It was easy to then practice mindful meditation, as soon as I practiced and became more proficient at staying present.
One easy tool is just to turn your awareness to how your body is feeling. Just notice. Flashbacks may occur, but both my Ts are teaching me not to get trapped by "the story" behind the flashbacks. if I can't quite manage the flashback without getting caught up in it, then I stop the quiet meditation, get up and physically move, practice getting present and grounded.
I'm really grateful to be working with mindfulness approaches for working through anxiety and other issues. I've noticed an incredible improvement, and am glad to have these developing skills.
I hope you experience good healing with this. If it's too much of an issue (especially in the beginning), it's not the end of the world... it's a very helpful tool for deep healing, but not the only one available.
I wish you good healing, and have hope you and your therapist do wonderfully together!
Warmly,
Deer