Do not let one doctor dismiss your concerns. Anyone at any age can have health issues. The answer that he gave you is a dismissive answer meaning that he does not believe you are ill and does not care to be bothered with your concerns. Physicians should be treated with respect. At the same time, remember that they are service providers. They work for you providing medical service and are therefore your employee.
Too many doctors forget that it is our money that pays the insurance premiums, employer sponsored programs where your employer pays a percentage is considered by your employer as part of your compensation. On top of that, you have to make a co-payment at the doctor's office. Medicare and government assistance programs were paid for by your tax dollars and those of your family and friends.
Bottom line is that it is just like many other people on this question have answered. See another doctor. Ask around, talk to people, try the American Heart Association or the American Lung Association. They can put you in touch with a doctor that will actually listen to your concerns.
I would start with a doctor that specializes in COPD
(Chronic Obstructive Pulmonary Disease
) because these doctors are doctors of Internal medicine. If the problem is your heart that they cannot treat, most likely they can diagnose it and refer you to a doctor that can treat it.
Good luck. I hope you do not have anything seriously wrong with you.