Deep calming breaths
I'm 34 weeks pregnant right now. All through this pregnancy, and even at my last two appointments (30 and 32 weeks) it has taken the nurse several minutes to find the heartbeat. This baby is apparently far squirmier and trickier than her old brother ever was (in utero). Even though I can feel her kick minutes before they look for the HB, my heart still sinks each time.
I guess what I'm saying is, it's normal - chances are very high that the baby is just fine and all of this stress isn't good for you. If I were you I'd have DH hide the doppler and only "let" me use it once a week. (Sometimes it helps to make someone else the enforcer).