Sara -
Have you had a stress test done on your heart yet? Perhaps you should rule out any medical reasons for the racing heart before you push yourself any further. Better safe than sorry, right?
I recently had some chest tightness with some pain, went to the heart doc and had a stress test done. He found nothing seriously wrong with my heart (called me the "champion of the treadmill"), so I thought perhaps the tightness was due to do bad air and my athlete's asthma, etc. A visit to the lung doc confirmed what I suspected, but I wasn't taking any chances, so I had it all checked out. Heart conditions are nothing to take lightly.
As far as the fitness, each one of us had to start at the bottom to get where we are now. We've all been out of shape at one point or another. So we definitely understand how hard it is to be patient with your training. You'll get there. 
Take care,
~BikeMomma
"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." --Albert Einstein