Having been in the same situation as Charles and Diana (around the same time too) where the marriage had "irretrievably broken down", with even trying marriage counseling, my counselor said that a lot of times, people that are in the position that I was in are called "married singles". Nothing really that was termed "togetherness" in a marriage yet still legally married.
When you're in that kind of a position, you have a choice to either make a life on your own without the marriage partner or suffer in silence to preserve the appearance of "being married". Psychologically, we're sentient beings that do not thrive in isolation without close human contact. The pandemic has proven this. I have to agree with Denville on this that Diana needed and wanted to have her own life.
Human contact and intimate close friendship does not always mean "affairs" or "trysts" or "cheating". I do believe that Charles and Camilla had such a close, confidential relationship that had endured through the years. It was the same with me. My husband now was my best friend when I was going through that "married single" time yet we didn't become romantically involved until 5 years after I'd divorced and had had a relationship with someone else in that 5 years.
I don't blame Diana either for exploring her world around her that she may have missed out on before she married at a young age. The truth remains that if a marriage is a healthy, happy one, there's no reason to look elsewhere for intimacy. Charles and Diana never did reach the close, intimate "best friend" stage and that's what is a solid foundation for any marriage.
Just my two cents of course.