It's obviously very common for those of us with health anxiety to completely bypass what would be more rational, likely causes of our symptoms and jump straight to the worst case scenario, so you need to do what Camel is suggesting, Angrry, and break it down into logistics.
What is the likelihood of the less catastrophic scenario? I'm talking about sinus infection, tension, something viral, or even anxiety.
What is the likelihood that it's not ANY of those things, which are more common, and that it's cancer, which is extremely rare?
How many times have you feared cancer before, or any terminal, catastrophic illness?
How many times were you right?
Look outside of your anxiety for a moment, and ask yourself these questions. What are the chances that this ONE time, in spite of never being right before, you are now?
Not very. So your job is to tell yourself from now until your appointment, until you're blue in the face if you have to, that the likelihood of your fears being unfounded are very, very high. Talk to yourself, yell at yourself if you have to. I am fine, I am safe, I am physically healthy, anxiety is a liar.
"Every day and in every way, things are getting better and better."
"Every cell in my body is in perfect health."
Speak them. Out loud. Every day, all day if you need to. Any time you freak out, or get anxious, or think "what if", speak to yourself. It helps, but I'm telling you, it's a constant, repetitive effort. You can't let lax and forget, or say eh it isn't working. You gotta keep doing it.
*hug*. Everything will be okay.