I too have these fears, sometimes to the extreme as I have an anxiety disorder. I highly recommend some meditation. I am still in the process of trying to do it regularly, but it does calm the mind. I actually went to a counsellor at the hospital when I was pregnant with DS as I didnt know how I was going to cope with the anxiety of having three.
I was sent to Calmbirth classes mainly to learn relaxation. I have just bought a mediation cd and have got a cd headset thing to listen to it in bed. It really does help, although Im not sure the anxiety ever goes. Another thing is being VERY conscious of negative thoughts, and making them into positives. xo