I want to take the time to write this out, because I felt the pain in your OP and also the shame you're feeling surrounding it.
I will start by saying I have never been through your specific situation and have no doubt it was worse than any of these, but these reflections and experiences from my own life might help you to put your own feelings into perspective.
My childhood was good in a lot of ways but also emotionally complicated, and I do think our childhoods play into how we learn to cope with rejection and abandonment. I have no doubt my interior belief that I had to be "good enough" to love became part of my story as an adult.
I was with my first love from age 14 to 17 and we were so in love and so happy and it was the first time I ever felt really safe. As I said my childhood was difficult, and he was the first person I think I felt fully loved and accepted by so my attachment to him was absolute - even though it was "young love".
When he headed off to university it just fell apart - partly that was my own fault as I was young and stupid. He dumped me and started dating someone much older than me (and much prettier) and it just broke me completely.
I know people are hit hard by first loves, but this really affected me mentally very deeply and even 30 years I see how much that affected me. I spent quite literally almost twenty years moping in one way or another. He married someone else around 15 years later and my heart broke all over again.
It consumed me, it really did. I didn't make an effort with anyone else, or form attachments at all, and with hindsight I don't think I could have played it any differently because I was just built that way and it's part of who I am.
So I hope that puts into perspective that being sad for six years after what you have been through is nowhere near as "embarrassing" as my own reaction to something comparatively small.
Seventeen years on, my second love was a (I thought) a very nice man who I was nowhere near as in love with. He was absolutely besotted with me, completely enraptured, and I remember being both happy I had found love again but a bit sad that it wasn't the same. He proposed to me, and I said yes, and looking back I was very happy as I thought I had found someone who would always love me.
He left me at the altar. Quite literally. No problems I knew about. No other woman. No explanation beyond a text message saying "I am sorry". He was just gone. Looking back, and this was now 13 years ago, I still do not understand WTF happened or why he did it to me. Had there been another woman it would have at least made sense. He never married, and no explanation was ever given to me.
And again, this rocked me to my core and I spent five years crying and dying inside but the feelings there were different. It wasn't about him per se, as I never loved him wildly like I loved the first - it was more than the life I had planned had vanished, that I was left essentially without a place to live and had to start again when I wasn't all that young anymore. It was excruciatingly publicly embarrassing and that changed me a lot too. I lost all my confidence and just retreated into myself.
Fast forward five years and I met my third love. This time I subconsciously went for someone who was unavailable on some level. On the surface, he was all the things I fancied in a man - clever, kind, funny - but underneath he was not ready for a relationship (he was newly divorced) but I entered into the relationship anyway and to cut a long story very short he ended up having an affair.
I can safely say that the mental pain of infidelity is beyond anything I have ever experienced, and I take my hat off to the posters here who have endured that when married for a long period - it is hell on earth and for certain is trauma. It screwed with my head in ways I can't describe and although we are very happily reconciled (after a great deal of hard work on his part), it will haunt me forever.
So while I haven't experienced your exact situation, I have known what it is like for someone to cheat on you, for someone you deeply love to disappear, for someone to blindside you my ripping your life away from you without your consent or any discussion and these are some of the most horrible experiences that a human can be forced to live with.
They do change you, and I actually don't think that feeling sad about it after six years is at all abnormal - it is a trauma, and a kind of deep grief that is with you for life in a lot of ways.
I think out reaction to break-ups generally is created by two factors: the first is how our own internal systems relate to loss and rejection (and mine were very fucked up) and the second is based on how we perceive the value of what is lost.
I had other small breakups, including one with the Father of my children who I didn't mention in the stories above because - the sad truth is -I did not love him nor rely on him and leaving was my own choice. In my mind, there was no surprise, no rejection, and frankly no loss.
Infidelity is another animal altogether. As is abandonment without discussion. Both these can cause you to get stuck, because there is no solution to the grief in your mind. It just ends up becoming a circle where you are looking for resolution that isn't easy to find.
Talk to other people who have experienced it. Anyone who has would not tell you that six years is a lot.
But in your case I also wonder if you maybe also still hold onto a perceived loss in your mind of a future you expected to have that was taken from you. There might be many elements to that - shared parenting, growing old together, financial support - and losing those things is deeply shattering, especially if you are blindsided.
I hope you are able to find a way to open your eyes to the possibilities of what can still come for you, as I know (and I have never met you) that your life is not tied to a man who did this - there is something else, something much more valuable, waiting to find you. I hope you can open your eyes to see it.
I think once you are happy or satisfied with the life you do have, the one you lost becomes less meaningful. Albeit, the things we truly love and lose are often always with us. He was clearly a twat and behaved appallingly, but there was a time you trusted, loved, and relied on him and it's very hard to reconcile that.
I read once that there was a framework developed by some doctor, which is often used in end-of-life setting but applies powerfully to any situation where a meaningful parting, closure, or release is needed - including relationships, family estrangement, or major transitions.
I used this around two years ago as a process to say finally goodbye to my old loves and embrace my current one, and felt a weight lift off me. The four things you must say to your ex H and ex marriage and ex life are these...
Thank you
(For what you gave, taught, or brought into my life)
I love you
(Or: I care about you / you matter to me - whatever fits honestly)
I’m sorry
(For anything unresolved, or hurt I caused - even unintentionally)
I forgive you
(For the things that hurt me, even if they were never acknowledged)
I think there's a process there where essentially you say goodbye to a life you believed was yours, and that you once loved, but does not belong to you anymore and from there you can begin the one you were meant to live.
I am now living the one I meant to live, and whilst I carry the scars and some residual pain from the infidelity, I would not change anything from the past that brought me here because my DP is now exactly the person I was meant to be with and whatever happens, happens.
I really wish the best for you, and sorry for the essay!