Although your doctor seems unconvinced, the first illness that I thought of when I read that you were fatigued and had headaches was anemia. I'm anemic and I take iron daily for more energy.
But as I read on that you were having heart palpitations, it seems more like the symptoms of Lyme disease. If so, you may also find that you have a skin rash somewhere.
maybe epsteinbar virus or cronic fatigue syndrome. your heart murmer would be unrelated.