It is physically impossible for your heart to "deteriorate" to the point of causing you seriously problems in the span of a few months. It takes years for say high blood pressure or a blockage to cause heart growth of affect the ejection fraction of the heart to the point of it being noticeable enough to cause problems.
the chances of having your heart stop (cardiac arrest) is 1/10 of 1% with the average age being 65. On top of which 75% of people who have cardiac arrest have previously had a heart attack, 80% have heart disease, and the average ejection fraction of someone who's had a cardiac arrest is 30 or lower. There is no chance what so ever for a normal heart that's had every test known available to progress to the point of being in danger of cardiac arrest in a few months. It would take years and years and years.
You're fine. Go out and enjoy being 19 and healthy.
To add to what the above poster said, obviously, you've had an EXTREMELY thorough work-up, if you had anything big and bad going on with your heart, it would have been discovered.
That being said, you obviously are still having a high amount of anxiety about heart issues, to the point where you're checking your pulse, and worrying about it frequently. That indicates that your anxiety needs addressed.
I would recommend letting your doctor know what's going on, and asking for a psych consult. A psychiatrist will be able to assess you and offer some treatment options, which could include therapy and possibly medications.
Please let us know how you're doing.
im hoping you can reply on my post nrsegirl... i really need some advice to be a positive thinker....