I couldn't read and not post. I too am pregnant with another bub after my DD was born last year at 20 weeks. Yesterday was her birth/death day and me again being 20 weeks preg at the moment made it quite surreal. I'm feeling a huge sense of release now that we have survived the first year, almost like I can breath again.
I don't really have any advice because I too am yet to deliver but I just wanted to wish you all the best and I really hope your birth goes well and your little one is born safe and healthy. Perhaps just let the tears flow? You have been on such an emotional roller coaster that it may be your body's way of "letting go" as your midwife says? Now that you are at the end of a pregnancy that this time will result in a baby you can keep, your body may be responding to that?
It is soooo understandable to be scared after what you have been through! Remember this new baby doesn't replace your other child, but becomes another addition to your family, and you have every right to continue to grieve for your first son while welcoming your new bub. I honestly don't know if it gets any easier, I wish I could tell you it did!! You're in my thoughts and I hope it truly goes well for you.
Not sure if I've been of any help, but I hope so!