How do you know? You are stating this as though it is fact. How do you know Charles was 'mildly fond of her'? How do you know that 'she didn't have his love'?
Diana most assuredly had it all. Even Camilla told her she did.
But what exactly did Diana want? The man was doing all he could to satisfy her demands. Nothing seemed to work. The upsets were never ending. What did she want?
How do you know he didn't? The man seemed to be crushed as his marriage did a spiral. How do you know Charles did not love Diana?
'Not right'? How 'not right'? She was destined to be the First Lady of the Land, the Queen. She had reached the social pinnacle of her class. She had a devoted husband, willing to work with her every need. She had privilege, wealth, and to some extent significant influence. Her job was to make her marriage work, professionally and privately. Her job was to respect the man that asked her to marry her, not throw him (and his family) under the bus when she needed barricades to protect her when her marital failures were coming to light.
Again, you accept this notion that she didn't have her husband's love. Where do you get that?
Diana willingly strayed from her marriage quite early. I personally think she always intended to do so, not as in premeditated, but as in that was the only kind of marriage she saw modeled to her. Her parents had lovers on-the-side, so would she. Why not? It was the way it was done. Nothing about this would surprise her.
I am going to suggest something few people consider: that it was
Charles that was shocked at the extent to which Diana had no intention of keeping to the marriage. It is Charles who demonstrates a steady one-partner loyalty. It is Diana who demonstrates a very ebullient love life. Diana found Charles boring and tedious and (fill in the blank). She moved on. She did not 'keep' what she had.