hi, your failure to drop in heartrate after exercise would seem to rule out anxiety.
Did you have a bad cold sometime before your problems started? It might have gotten into the heart muscle.
Otherwise you'd think about structural abnormalities, which would likely be genetic and so you'd look at what happened to older blood relatives.
This is something you should discuss with your parents, even if you are out on your own. Then, it would be best to see a doctor and get a general check-up.
You should first be very honest with yourself about what you've been eating and doing to see if there is something you can identify that may be causing all or part of your symptoms.
If you can't afford to see a doctor, contact your county or state social services department to see how they can help. Be creative if you look for help from your county/state, I'm not sure what they will call services they offer - I am confident help is available if needed.