Have the cardiologist review the echocardiogram, in particular assess the severity of the regurgitation in the valves. You may need to undergo a TEE, to better assess the amount of leakage in one of these valves. The tachycardia may be secondary to left atrial dilatation from the regurgitant valves.
It sounds like you could have POTS. This is not fatal, but you do need a doctor that can treat it. dinet.org. & NDRF.org.
Most of POTS patients have MVP and other valve regurgitation. The autonomic nerves system is out of balance (ANS).
Hope you can find help
I have experienced similar symptoms, and can relate to them being scary. (Am also a mum in my 30's). If you are concerned that your doctor is missing something or not taking things as seriously as you'd like, it might help set your mind more at rest if you seek a second opinion. Sometimes these symptoms can be problematic (unfortunately) yet benign (fortunately)... which I suppose is better than problematic and sinister. Good luck with your work up - I hope it turns out to be something that can be fixed, or something that goes away.