Ok so this is now making sense. Your first experience of someone being seriously ill was your mum not dealing with it. The reaction she modelled for you was that it was catastrophic, world ending, something to be terrified of. Her reaction was probably quite traumatic for you at 16, especially when you were already dealing with your own feelings about it. Did your relative recover?
Then losing your dog, so unexpectedly, sounds like he went downhill fast. This may have created in your mind a belief that any minor illness or symptoms could be something sinister that could potentially cause you to go downhill at any time.
Logically I'm sure you know that statistically this is unlikely but these two experiences have in effect created a major trigger for your anxiety. You have trauma there and are being constantly triggered. This is of course just my theory and musings, I may be wrong, I may be bang on the money.
As someone who has CPTSD I have real sympathy for this if its what is happening but since triggers can't be avoided, it's a case of having to deal with them. Have you ever heard of EMDR therapy? I think it could be very useful to you, especially in conjunction with some CBT.
While you look into what is available, wait for referrals etc it might be worth looking at methods of breaking out of a spiral.
You have to consciously prevent yourself going there. I tell myself out loud to stop it. 'Of course they don't hate you, Balloon, they don't even know you, don't be so fucking ridiculous. Drink your coffee and stop thinking before you give yourself a headache.'
Tell the paranoid part of your brain to shush, not now, you're busy and its chatting shit anyway. Reason with yourself aloud, like we're doing here. No, you don't have xyz. This is why.
Distraction techniques. I sometimes play the kids games if I'm anxious. Who can focus on a panic attack when this FUCKING PIKACHU is getting carried by the whole team for 2 towns and still goes down in one hit? Not that Pokemon gives me road rage or anything...
Grounding is another good one to start with. Count 5 things you can See. 4 things you can Hear. 3 things you can Feel. 2 things you can smell. 1 thing you can Taste.
Carry chewing gum. Or Smints or something. Burst of taste or even loud music can sometimes halt the mental merry go round.
Patting helps. Use your hand on your collarbone and pat firmly and slowly. It simulates a slower heartbeat and your own will often slow to a similar rhythm.
Be around animals. Dogs, horses whatever, they're very soothing presences. Horses in particular I find calming. Riding lessons changed my life. Though I am now known as She Who Has the Treats so get followed across the field by a procession of horses like the pied piper. Or as another poster suggests, could you get a dog?
You can absolutely change things. You just have to go into practical mode and set the balls rolling.