Lost baby at 21 weeks

I got back from the hospital yesterday after delivering my baby girl who passed away at 21 weeks. I'm feeling extremely broken, and not sure how I'm going to move on from this. I had my anatomy scan last Tuesday and everything looked great. I started not feeling movement after that. After a few days went by I started to get worried. Finally the following Wednesday I went to triage first thing in the morning and that's when they told me she was gone. I got induced and delivered her the next day. We got to see and hold her and say goodbye. The process was so hard and my husband and I are devastated. We have a 1.5 year old son I want to be strong for, but I feel so sad knowing I'll never hold my baby girl. We were only 4 months away, and now I don't know what the future holds. When she arrived the doctors found a tight knot in the umbilical cord, and there's a good chance that was the cause. I know that it was just a freak thing that you can't do anything about or prevent, but I just can't help but wonder if there's something I should have done differently. I cant stop replaying the last week and a half over in my head. I know it's all still fresh but I'm so scared that I'll never be able to get over this.