I doubt anyone here can give you an answer, but you may think back over the last few years about anything you have done that could be unhealthy. If you come up with anything you have a suspect, if there is nothing and the "innocent heart murmur" diagnosis was correct it is my guess that nothing much has change in the 6 years since. Now, my view is based on the knowledge and experience that many people suffer from some heart murmur, many may be diagnosed by nothing more than the stethoscope, maybe not even an EKG. I may have had a murmur at an early age, I was in the Navy, but that may have been before the military paid any attention to such things. In my case I was diagnosed with a leaky heart valve and then later had to undergo surgery to repair it, this all came about when I was in my 60s. I am not trying to diagnose your situation, just giving you some thoughts that may help you remain optimistic.
Good luck and I wish you success in your life goals,