Is it possible that it is not your heart, but something else in your chest? Maybe heartburn or something like asthma?
I'd go back to see your doctor and ask them to look into it more. Maybe have a bit of a google of things like indegestion and lung/ breathing conditions too and see if you can think of any other symptoms or triggers.
This is something that you should keep pushing with. You shouldn't be having regular pains at a young age and if your doctor is dismissing it i'd try to see another one.