I'm sorry for your loss, because it is a loss. What you are doing is grieving. This is normal and right.
I was not in our position, but I think my experience is close enough that it may help you.
I had DS in 2004, no problems. I became pregnant again, realised on Christmas Eve and worked out baby would be 2 years 2 weeks younger than DS. I suffered lots of early bleeding and saw the early pregnancy unit, was scanned, no definitive answer, scanned again a week later and told all ok but my dates were wrong.
I never felt the same as I had with DS.
I went for 13 week scan to discover I had a missed miscarriage. As we have private insurance I chose to have an ERPC, they ran tests and told us that the baby had a trisomy disorder that was incompatible with life. This made my situation much easier to bear. It was Edwards syndrome.
We did not have to make your difficult decision to terminate as nature took that out of our hands, we had however already discussed what we would do as DH has a cousin with Downs.
After this I went on to have 2 healthy DDs and now have a family of 3 delightful children. What you have had to deal with is really hard, it is natural to think about anniversaries and due dates. I was lucky because I was pregnant with DD1 before my due date, and she was born soon after the anniversary of the dreadful scan.
I felt lucky that I already had DS so I knew my body could do this, you are still young (10 years younger than I was when I had DS).
Men tend not to discuss their feelings, but I have always been thankful that I had an early MC rather than a still birth or disabled child.
Things will get easier. As they say on MN this too will pass. Good luck.