If you have had stress tests, ecg/ekgs and blood work from more than 1 doctor and all is fine, the probability of your having actual heart problems is very small.
Sometimes panic and anxiety can present itself when people are insecure about their health. If you are really insecure about your health, to protect you from harm, your mind may produce symptoms that basically make it difficult for you to exert yourself.
You may need to get help in the form of walking short distances with a therapist, and increasing the distances as you overcome your anxiety.
The thing that you do NOT want to do is become completely sedentary. It can make panic and anxiety worse. One of the most beneficial things for anxiety is aerobic exercise for at least 30 minutes 4 or 5 times a week.
Good Luck!