Hi there Treacle.
I’m so sorry to read this. Anxiety is so debilitating, and health anxiety added to that sounds exhausting. I’d love to offer you an answer - but I don’t know that I have one! I can only offer my own thoughts about it, which may be useful or not.
I see you’ve accesses various therapies, all of which can be helpful - but only it’s the right therapy with the right therapist for you, at the right time… I’m going to assume each option you accessed only gave you a set number of sessions, which can add to the anxiety, I think. Then there’s the anxiety of, “that didn’t work, will this work?”, maybe followed by, “what will work, what’s wrong with me, why can’t I make this work”, or similar, perhaps.
Trouble is, the brain creates the anxiety to keep us safe from a perceived threat. It’s like the fight-flight-freeze reaction has gone into overdrive, and that comes from a deep, primal part of the brain that is hard to get a grip of! Plus, with a cancer diagnosis, it’s easy to think something like, “well, I got cancer, so I could get anything, so I’m not safe, and it could all be bad, and…and…and….”.
For context, I’ve struggled with PTSD around “something bad will happen someone’s going to die”, for many years. Then, I got cancer - so yes, I’m on high alert to what’s happening in my body. (Currently 18 months post-treatment and all good 
). And in those 18 months, two friends have died from cancer (not breast), and another from sudden and unexpected heart failure. Recently, my sister was diagnosed with motor neurone disease. So, I absolutely know that the worst really can happen.
Thing is, I also know that 99% of my fears and anxieties and “what ifs” are not going to happen. They aren’t likely to happen. If they do happen, there’s not much I can do about it. I cannot prevent the random crap that life throws at us. I wish I could. So I sort of CBT myself - when a friend stays over and sleeps late, by about 9am part of my brain says “well that’s it, she’s dead, what will I do?”. And then I give that part of brain a slap and tell it to stop being so stupid, because whilst I know it is possible, I know it is incredibly unlikely that she’s died. And if she has - well, naff all I can do about it, and I know I could deal with it because I’ve dealt with worse. But it’s so very unlikely that I need to just get on with what I’m doing and wait for her to wake up.
If you’ve not tried CBT, maybe it’s worth a go.
The other thing I try to remind myself of, and follow, is that I refuse to waste my life on anxiety. Not everyone gets the option to get up and live freely every day, and there is often neither rhyme nor reason to who gets ill and who doesn’t. I get why you choose to stay home all the time, but don’t isolate yourself forever.
Sorry I’ve rambled on, I have no idea whether anything I said might be helpful at all, but I’m happy to talk and help if I can.
Xx