1. Use probability.
If you have been checked out medically and you get the all clear, it is easy to slide back into panic thinking they missed it or you are that 1% of the population that has a different presentation of this illness. Instead of the focus being on the 1%, let's focus on the 99% chance that everything is fine! If someone told me, hey there's a 99% chance you will win the lottery if you go play right now, I wouldn't reply with, "Nah because there is a 1% chance I won't so I'll save my dollar." Anything is possible but is it probable?
2. Calm your body.
When our mind is racing and we feel we cannot control our thoughts it is helpful to relax the body. This could be through massage, a hot bath, sauna, swimming, etc. Often when our body is physically relaxed our mind will understand this and processes this as "oh the body is relaxed so I am not in any danger, I can calm down too" which stops the dumping of adrenaline and cortisol into our bodies, helping our symptoms improve.
3. Recognize it could be anxiety.
For some reason people are more willing to see a doctor than a mental health therapist. They are hoping it is medical instead of our emotions. Anxiety causes strong physical symptoms that can be interpreted as medical illness (heart racing, palpitations, breathing difficulty, headaches, stomaches, dizziness, visual difficulty, to name a few). These are very real symptoms however they are not always caused by medical reasons and often caused by our anxiety and thoughts about the symptoms. This is why medical reassurance only makes you feel better in the short term. Consider seeing a therapist to help you learn how to work through the health anxiety and your symptoms will improve.