I'm currently going through a pretty intense episode of mortality anxiety with looping obsessive thoughts. I worry about getting cancer in the far future and when i see people who die young/middle aged/60s or 70s it convinces me I will too. I feel that if I was guaranteed to die at a very old age I'd be okay.
I have to keep trying to remember that it's just anxiety. I often get cancer dreams where I'm diagnosed and then feel like I have it.
The thing I try to remember is that if I had cancer, I would know about it. My mum told me that the symptoms are VERY obvious and not only is it rare in young people, I would KNOW if I had, say, a brain tumour because I'd be having intense headaches, blurry vision and feel very poorly indeed.
Another thing that helps is to be able to relax and accept that you feel the way you do about your health. Don't fight the thoughts, accept that you have health anxiety. Having health anxiety won't cause you to die or become ill from stress. Stress and anxiety simply cannot kill you. Often people who have health anxiety feel the way they do because they have a lack of control in their personal life, very true for me because of my family situation.
Drinking chamomile tea, sitting with people you trust and hugging them and having them hold you close, getting into a good bedtime routine and staying off screens an hour before bed are all very helpful to me during this time.
SSRI antidepressants, such as sertraline (which i've started a few days ago! wish me luck!) have been known to help obsessive anxious thoughts. I know people who have taken sertraline have found it very very helpful and stopped obsessive thoughts from triggering panic, allowing a clearer mind. If you haven't considered SSRIs, they are worth a try. Stay away from benzodiazapenes (such as diazepam) as they cause horrific withdrawals and are actually damaging drugs. Beta-blockers also help reduce increased heart rate with panic attacks and can ease physical symptoms. I take them for my heart (I have a non-life threatening heart condition), but personally they don't take away my anxious thoughts.