Anxiety! I keep getting stroke symptoms down one side and I panic. I get it so often and think I'm having a stroke every time. How to stop worrying?

Seek psychotherapy. Given your age, the likelihood of stroke is low given that you have undergone brain MRI, echo, holter monitor, etc. Therefore, see your psychiatrist or psychotherapist to help treat your anxiety. Consider supplementing any psychotropic or anxiolytic medications with relaxation techniques.
See a physician ASAP. You need to have a comprehensive examination by a physician ASAP; Any possible stroke diagnosis has to be ruled out; Once you are told is not due to a stroke or another physical disease, you may consider an anxiety disorder; This has to be diagnosed by a Psychiatrist in a comprehensive evaluation;