D,
The reality is there's little you can do to keep your mind from going ballistic while waiting either for surgery or the diagnosis that it is necessary.
Breathing, meditating, diversion techniques, etc. are all useful but the stress is ultimately inescapable. I know of no one who can turn their brains off at will. Just know it's something we have all lived through.
If you think back to anticipated scolastic tests or even the 3:00 bell or birthday parties or a special outing or vacation, the anticipation for all of those made you want to jump out of your skin. You could actually watch the clock tick second by second. It's the price we have to pay for the ability to think
But by all means, try breathing, meditating....
Good luck!!!!! Dale