With my first two (successful) pregnancies, I got one of those fetal Dopplers to use at home. I was able to find the heartbeat at 10 weeks, but it took over 30 minutes both times. It's very hard to get it that early! I've even had friends whose doctors couldn't hear it at 12 weeks in the office, and everything was fine. Where your placenta is and whether your uterus is tilted can also have an effect on it. By your next appointment (14 weeks?) they should be able to find it, and I can almost guarantee that they will send you for an ultrasound if they still have difficulty getting it.
I understand your worry--I've been there! I, too, had a miscarriage for my first pregnancy and then conceived again 2 months later! I'm now on my fourth successful pregnancy (15 weeks), with no other miscarriages besides that first one. Even so, I can't help but feeling anxious sometimes. With this one and my last, it took the doctors more than a couple minutes to finds the heartbeat at 12 weeks, so what you're going through is quite normal!!! Hang in there, and I pray that in another month, you will hear that beautifully beating heart!!!!!