Kristy,
I'll bet it is tough, my sister went through a similar situation as you and with her next pregnancy she worried herself sick with the fears you are experiencing. I think the doppler thing may be a good idea, it may really help you get through the times when you are especially anxious about it. But, also...look at your other children and remember you have living proof that the odds are greatly in your favor...they really, really are.
hugs,
wendy