I don't know what she could possibly have on him that he couldn't make go away with his practically endless money. Seems strange if she was blackmailing him that he'd choose to be with her rather than just send lawyers after her.
I definitely think he was looking for a way out of his relationship with Millie and if he's someone who doesn't particularly like talking through his feelings then "I've met someone else" is probably an easier discussion than "you don't make me happy anymore".
The only argument for them not to be what they seem is that she seems like a
fool but he doesn't. But I think some couples just are that different and from the outside we don't know how they are with each other.
I love speculating on the hypotheticals but my strong feeling on this one is that the relationship is genuine. I do think there may be something in what
@secrethush says about her potentially manipulating him early on in their relationship. But right now I think they're probably doing OK and are happy together.
In the past when he's had personal problems it's been obvious on the pitch because he kept making mistakes. Maybe he's just more mature now and able to separate the personal from the professional, but he seems like he's in a pretty good place at work in terms of being reliable and handling whatever's thrown at him, and I think that probably points to his personal life being relatively stable.