You can try positive affirmations as well or maybe Google meditation techniques.
I def understand where ur coming from. I'm 18wks and was worried all the way up until my us yesterday, when I saw my baby just hangin out wiggling around with a nice strong heartbeat. Getting the doppler should ease ur concern. I would've liked to get one, but didn't cuz now I can feel my baby moving.
It is hard not to worry when pregnant, however worrying can bring lots of unecessary stress. Try to keep busy, spent time with friends and family and enjoy the blessing of being pregnant.
Even though the doctors disagree with purchasing one I bought myself a fetal heart doppler so I can listen to my baby's heart beat at home. I guess they don't want people to freak out constantly if they don't hear it. Don't really have any other suggestions sorry