Quoting Sour Daisy:" You have to defer those thoughts when they start up. Try an catch yourself when you're thinking about ... [snip!] ... you are to see your bran new babies little face for the first time It takes time and practice, but it really helps."
this. when i was pregnant with my son i experienced the same thing and when it happened i would train myself to think happy or hurry and go do something that i knew would take my brain away from where it was. i would tell myself i was just over thinking and it was my brain playing jokes on me that seemed to help and also going outside or going on a walk(probably hard to do with as far along as you are) or maybe just sitting outside. i hope everything gets easier for you. anxiety is no fun