If you can't stop thinking about this, and it doesn't sound like you can, try to think about it differently.

Do what diagnosticians do. Look for the pattern, not at any one symptom. Do they fit together? A two-day rash is not alarming if it's not part of a pattern.

Play the odds. Factor in your age, gender, family history...what are the odds you have X? They are probably not very high for most conditions you're Googling.

Obsess not in the sense of sitting around crying but write every single odd symptom down. Leg itches? Write it down. When, how long, time of day, severity, possible factors like you just ate, etc. Write down what you eat, how long you slept, what exercise you did and for how long. That way you can see if there's a pattern.

Modern medicine is far from perfect, but there are treatments for most conditions. Cancer, for instance. The treatment may not be pleasant, and it may ultimately fail, but having condition X is far from being a death sentence. And most conditions are more treatable the earlier they are caught - and you are obviously on top of any possible symptoms.

And ask yourself what the cause of the anxiety is. Fear of death/dying/hospitals/doctors/losing control? A passive death wish?