Hello Ruby I hope your OH is looking after you.
From experience, if you have had a difficult time in the past, when you are stressed or anxious it seems to make your brain remember past difficulties more. So feeling this way doesn't necessarily mean you haven't "moved on". Then you have pregnancy hormones and natural anxiety about your two wee babies on top, it's a tough mix to swallow.
Make sure that you have someone you can share these feelings with, and that your OH knows what kind of support you need from him (I'm guessing a lot of reassurance will help).
Bad dreams can be worrying, but try not to think of them as a projection of your daytime thoughts. It's more like your brain filing pieces of paper away, and sometimes your emotions pick up on one phrase on the paper and go overboard on it.
Do try the mindfulness, it will help you to get a little peace and distance from your worries. Remember that you don't need to fix everything (anything!?), you just need to keep plugging on, one day at a time, one foot in front of the other, and trust that things will work out.
It might be worth considering having a few counselling sessions to get some things off your chest, especially with a CBT specialist.
Thinking of you, and hoping you can put your mind at rest, even if only for a little while. 