I've read your posts on early March and and most sorry to hear that you not only have a trauma or traumas in your past but that your GP is talking about cancer (sorry I could not find your other posts).
Both are terrible things to have to live with and of course support is vital. I remember you saying My family are beautiful but not best placed to support me, and wonder if you could say a little more, don't they understand, or do words fail them, or perhaps they live elsewhere?
I'm not being nosy, the more idea we have of the things you have to face, the more specific can be our replies and suggestions.
Being told by a GP you normally get on with that cancer tests are needed would very much have been a shock, and I'm sure you don't want it have to go through the whole anxiety ridden process. However if matters are minor finding out early may be of great benefit.
I know this is a logical answer, and logic is not always persuasive. Do you think it might be a good thing to talk to your GP? Underline the fact of simply the cancer tests being proposed is itself traumatic, and when added to your previos trauma could have a most undesirable effect?
There are trauma informed practitioners, both psychologists and psychiatrists. It is a specialized area, but such practitioners are reasonably common. Now that you can video-link to them as part of the new tele-healht measures it might be possible to find one for you - again I'd suggest talking about this with your GP.
As for seeing it as an either/or situation, perhaps with the right support you may have your tests and at the same time receive psychological support both for the trauma and also the tests. At least that way you will not have the worry of the unknown hanging over you, while you will have the knowledge you have made a decision and are more in control.
Please let us know what you think