I absolutely worry about my body just like you're worrying about yours right now. All the "what ifs". Us poor humans find it so easy to think the worst when pretty much every time it's not the worst.
Did you find out what was the cause of the dark nail or your mouth problems? Were the two related? It's great that dermatology were able to set your mind at ease. I don't know what the 2 week rule is if you'd be able to enlighten me.
CBT should be able to help you if you're a worrier when it comes to health. One of the things I would speak to my therapist about is how I worry so much about my health. "Aah I'm dying, aah I have this horrible disease, aah this'll happen that'll happen" Even when you
know your anxious mind is in overdrive the feeling of doom can still feel very real.
Let the doctor diagnose whether you have leukaemia or not. If you're not a doctor, you can't make that diagnosis. If the doctor gives you the all clear, let that calm you just as the dermatologist was able to calm you with the dark line on your nail.