It's often the vagus nerve connection. I know a possible way to short-circuit it.
When you feel it starting, Say quietly to yourself, "Oh, my anxiety is stimulating my vagus nerve again."
Wait 4 seconds, and then say this, quietly to yourself:
"Beta-adrenergic receptors on the afferent arm of the vagus nerve detect release of epinephrine from the adrenal gland. This acts as a “something important just happened” signal that activates noradrenergic projection neurons in the locus coeruleus via the nucleus of the solitary tract."
Wait 4 more seconds and then say this quietly to yourself:
"While the amygdala is most closely associated with the acquisition of fear, plasticity within the amygdala is also critical for extinction learning."
Often, this helps.
You can print this on a card and read from it, but it works best if you memorize it. It's O.K. to mispronounce, as long as you say the words in order.
Give it a try.