I've had terrible health anxiety too. It's awful. In fact I nearly didn't click on your thread as often just seeing things about health issues - especially cancer - can start me panicking. Mine started when I completely convinced myself I had bone cancer and would be dead in a matter of months. I had the worst panic attack I've ever experienced. My DH and my mum tried to reason with me and even though I knew they were being logical, I couldn't believe a word they said. My mum came with me to the GP where I was just a sobbing wreck throughout the appointment and was referred for CBT. Initially I thought cbt would be a waste of time, as no one can guarantee I'll never get cancer (or any other horrible illness) so how could I possibly stop worrying about it? However after a year's worth of sessions, I am in control of it. To an extent, the anxiety is still there at the back of my mind - I don't think it will ever go away - but the cbt helped me immensely with learning how to rationalise my thoughts and not let my
mind run away with itself.
I also used an app called "Mindfulness daily" which help to keep you focussed on the here & now, and to let go of worries that you can't do anything about.
A couple of useful things I learnt from my sessions that you could try when you are in the middle of a panic attack -
Square breathing. So picture a square in you're mind, and "trace" it's outline as you breathe. So across the top of the square, breathe in and think of something that makes you feel happy .. it could be a person, a place, a colour; anything as long as it has a positive association for you. Then hold your breathe as you trace down the side of the square. Trace along the bottom of the square and breathe out whilst thinking about the negative thoughts that you want to let go of. Again hold your breath as you trace the final side of the square. (I sure if you Google square breathing it may give you a better explanation than me!)
Another one is 54321 to make you focus on the present moment and take your mind away from your anxieties. So name 5 things you can see; 4 things you can hear; 3 things you can touch; 2 things you can smell and 1 thing you can taste.
Oh and having a couple of positive affirmations to repeat to yourself can be helpful too. Anything that you feel is appropriate to your specific situation. Examples that were suggested by my therapist include things like "All is well, right here, right now" and "I am safe & well and have all that I need". I often use versions of these to try and calm myself when the need arises.
The MIND website has some useful stuff on it too.