Basically, I don't think Diana ever totally lost her fantasy "Cartland" beliefs in how being "in love" should be. There was no man on this earth that could really fulfill those expectations that were straight out of fiction.
With each and every male she found herself attracted to, we saw the same behavior as we did when she was with Charles. She wanted that pedestal to be on and admired and total devotion from the man she was with and resented those little, everyday normal things that people do such as duties and responsibilities and careers and even wives from standing in her way. She resented the Prince of Wales role as it took Charles away from her and she resented anyone vaguely close to Charles. She resented Hewitt's military postings for the same reason and tried to get him out of it. As much as she admired Khan's work as a cardiologist, she wanted that work to be on her terms somewhere else. She threatened, harassed and pursued Hoare like a stalker to get him to fall in line with how she wanted things to be.
Diana had definite ideas of how she should be loved by the man she's involved with but I don't think she ever learned how to be loving towards a partner in the real sense of the word. To support someone, to put someone else before herself in a relationship and to communicate and compromise just wasn't in her wheelhouse. She didn't know how. Diana wanted and needed love but actually had no real experience in what a true, deep abiding love for another person was really like. It remained as something on a very superficial level with her until the day she died. Love, to Diana, what how someone expressed themselves towards her and that's how she measured it and manipulated it to reflect how she wanted it to be. It doesn't work that way and Diana found disappointment after disappointment. Should she have lived, I imagine that sooner rather than later, she'd find herself disappointed with Dodi and started to look elsewhere for what she felt she needed.
Maybe the right one was out there for Diana that she was looking for but sadly, she never found him.