Health anxiety can be crippling. Have you asked your GP for referral to therapy to help with it? Or you could look into finding it yourself if your budget allows. Hypnosis is another possibility that might be able to train your brain to think of something else when the obsessional thinking sets in.
It's very difficult for others to advise, as we don't know your circumstances. Things like U3A are great if there is one you can get to easily, for instance. If there's not, or if you don't have access to transport, it is pointless to suggest them.
Sometimes, health or other 'specific' anxieties crop up because the sufferer is (inadvertently) channeling other worries into one more defined anxiety, and it goes into overdrive. It's easier (ironically) to worry about illness than to deal mentally with more 'free floating' anxieties, but then the health anxiety feeds on itself. It's not helped by the fact that the symptoms of stress ca mimic the symptoms of feared diseases.
One thing that might work is to jot down symptoms as they arise, and date them. Then, when they recur you can look back and remember that you've had that before but got better, and the 'real' disease would only have got worse. Repeating this over and over will eventually make you realise you are not physically ill, and will switch off that reflex in your brain.
In my 30s I suffered with HA, and the above worked for me. It takes a while, as you need the ability to 'look back' to convince yourself that it really is all in your mind.
There is a series of books by Dr Claire Weekes, which deal with this sort of thing. They are designed to be read through brain fog, so you need to bear with the repetition and very 'easy' way of writing, but they do reiterate the message that anxiety causes real physical symptoms that mimic those of diseases. Many people don't recognise this, and assume that sufferers are malingerers or hypochondriacs like Dot Cotton in Eastenders. That is not the case, and a good GP will recognise HA as a real condition in itself.
You have my sympathy - as I say, the ridicule from some quarters is almost as bad as the condition - but even without that, you are going through the same feelings as someone who does have the diseases you imagine. The fact that you don't have them is neither here nor there - in your head you do, so you feel the same feelings. Please try the jotting down thing. It's very easy to do, and can be very effective. 