Wannamoveforward,
Having just read your entire thread, I am blown away by your last post. You have nailed exactly what the issue is, once the dust of D-Day has settled.
I think it is the hardest part of recovery (once you stop reeling in agony from the initial trauma, anyway), and I suspect it takes a
long time to work through.
I think that the reason it is so hard, and the reason it takes so long (at least for me) is that it involves developing a deep sense of empathy. Hard to empathize with somebody's pain when it's what THEY did that is causing the worst pain YOU'VE ever felt in your life. Only a long period of loving and being loved and feeling truly safe with each other can permit such empathy. Still working on that.
But that's just my take.
As far are the anniversary thing, I can really relate to your feelings about it. I discovered FWH's A 5 days before our anniversary, although I hadn't yet told him I knew. I sent him a dozen red roses at the office, as an expression of care. Do you know what he did with them? TOOK THEM TO OW's HOUSE AND GAVE THEM TO HER!!!

That memory causes a real problem for me at anniversary time, but less this year than last.
So, the passing of time is a factor too, when you realize he chose YOU in the end. Now the flowers he gave her mean so much less than the fact that he dumped her and truly committed fully to ME. Who knows where SHE is, and who CARES? It feels GOOD to be able to say that, and to know it.
You are still so early in recovery to be seeing these things. Be patient with yourself. I think you're doing great.
Right Here Waiting