Can someone please advise me how to work through my latest fear, from an anxiety point of view

It feels as though my fight or flight response is permanently turned on, always in a state of panic. I've been like this before and it is so hard to get out of it.

I'm going from one fear to another and my latest one is now my armpit. I noticed in the shower this evening that my left armpit is larger than the right one, as in it feels like there's something bigger in that one. It doesn't feel like a lump, but whatever is in there is bigger on the left side, if that even makes sense.
I know we're not symmetrical, and it's not like it's a hard lump I can feel. But every time I try to talk myself down, it's as if the panic just takes over and won't allow me to be ok with it.

For anyone who has beaten or is in control of their HA, how do you deal with issues like this? How do you stop things snowballing and just learn to get on with your life?