Hi OP, I mean this in the nicest possible way, but no one will be able to talk you down - you're in the grip of your anxiety and any reassurance you do get will be temporary before the anxiety takes hold again. I say this as someone with health anxiety who has also been convinced that I have numerous cancers/MND in the past.
In my case the twitches were anxiety and stress related - I was extremely stressed and overworked, tired, dehydrated, not looking after myself basically, and my anxiety was really uncontrolled. I then became worried I had MND and the twitches got worse because my stress/anxiety levels got even higher, so it became a vicious circle. I left the stressful job for one I'm much happier in and the twitches have all but gone (bar ordinary muscle spasms that I'd never have been aware of before). Your GP has already suggested anxiety as the cause, though (which is most likely) and you're unable to believe them, a trained medical professional, so you won't believe me because I'm a random on the Internet with no medical knowledge (and that's fine, I get it).
Logically, you must know it's extremely unlike to be MND, but your anxiety is masking the logical part of your brain. I suspect you won't be happy until you have the full battery of tests to prove absolutely that you don't have it - but your GP doesn't believe that you do, so you're unlikely to get them. By all means speak to them again, but I suspect that even that won't really reassure you, having been there myself.
Are you able to speak to the professional you're doing CBT with to help you manage your response to this? I went through IAPT and did CBT after my health anxiety nearly ended my marriage. It's the best and most freeing thing I have ever done for myself. I feel like I'm finally living a whole life again, so do stick with it, or if CBT isn't working for you then look at other options.
Sorry if that seems harsh, but just to say that I've been exactly where you are and it is possible for things to improve. Wishing you all the best.