I don't know that my opinion was changed by an event
Or even that it has changed.
I remember watching her wedding and thinking "wow, what an event! I wouldn't be her for anything, anything at all."
I watched her stumble and recover, trying to find her spot in the inflexible Universe into which she had married Thought it was cool that she was having children early on not just because, clearly, that was her role - but also because it would mean that she would still be young when the children were grown, and she could herself grow up and mature during that journey of parenthood.
I always saw her as someone who did what she could with what she had - I truly never envied her wealth or titles.
I had mixed feelings about Charles but mostly figured he was planning on using her for what he needed but that he was just too old and set in his ways to have much use for her beyond that. He was only 32 but, as he was frequently called, a "young fogey."
From my viewpoint, there was a calculated cynicism to her selection, right down to Phillip's comment that maybe she'd breed some height into the line. And I thought to myself, if she loves Charles even a little bit at the beginning, she will come to bitterly regret it.
I think he did feel warmly toward her, affectionate: bits and scraps of feelings. And that rather than seeing those hors d'oeuvres as being all he could possible dish out onto her plate, she thought they were an appetizer or salad course, and kept waiting for the entrée. It never came. Other things arrived at the table, but never the main course. And it was as though she was able to watch others feast on their entrée, right there at the same table.
But I did have some hopes that as both grew older, and age and maturity closed the gap between them, that they would find a comfortable spot in the middle somewhere.
If Diana hadn't married Charles, I think she would have married some titled chap, some landed member of the same aristo set, house in the country/townhouse or flat in London, birthed the heir and spare to him, moved in the same circles into which she was born.
Ironically, it was being Princess of Wales that was the making and breaking of her. To have married much like the other members of her class would not have raised huge expectations in her. Marrying a Royal, entering those circles with all doors open, traveling in sophisticated styles, having the world become a treasure trove of discoveries....was like a Pandora's box. It brought into play expectations far beyond anything that really anyone can dream. So much was given, so much put on her plate...but she still wanted that entrée that she saw others being served, but was denied to her.
So my opinion was not related to subsequent or specific events, but more as a mosaic of the situation over time.
My opinion is that the failure of the marriage is attributable to both parties, but that of the two, Charles had the better tools and equipment - the maturity - with which to cope and guide; thus, the failure in my opinion lies more heavily with him.
YMMV.