ILW2, your experience reminded me of my own recent experience with my fiance and our (now ex) girlfriend. We thought and talked about it a lot and I think one of the reasons was a difference in levels of comfort. When I am with my fiance I am the most able to be vulnerable because I feel safe. It's kind of backwards from how I expected it would be, but I think since I know I am safe with him, I allow myself to feel fears/flashbacks etc around him. With our girlfriend, while I was comfortable enough with her to have sex with her, it wasn't the same kind of super safe environment because she was a new person. There was of course all the NRE and accompanying excitement / anxiety that comes with a new partner and I think that can temporarily block out the tendency to have those negative feelings because you can only feel so many things at once. Unfortunately it does seem unfair to my fiance who has helped me get to the point where I am, but it is because I love and trust him so much, and trust that he will not hurt me when I am in a vulnerable state, that I can show that part of myself around him. (It turned out that our girlfriend was very negative and unkind about the things I did open up about, but that's another story)
Anyway, it sounds like everyone involved is doing a great job communicating and I wish you all the best.