You're right she is scared, but that's not the prime motivator for her actions at that moment. The two choices are between her standing up to Anthony because she refuses to accept his version of events or giving in and believing that her own interpretation of the events in the office was at fault. Neither of those choices was primarily affected by fear or her lack of it, but by her willingness or resistance to accept Anthony's lies. On the one hand, she's saying enough is enough I'm not going to fall for this any more and even though I'm scared ( when Anthony slaps her), I'm going to have the courage to stand up to this intimidation, because I believe my interpretation of events. Whilst with the other she just falls back to her usual acceptance of whatever spin Anthony puts on events, not because she's afraid ( she's afraid in both cases), but because Anthony has abused her over a long period of time, to the point she's become conditioned to accepting whatever he says. In this instance she ignores her instincts, which are screaming out to her that Anthony is giving her a load of bull, and just decides to believe him. If fear was her prime motivator at this point, she wouldn't be able to resist, as she does for the other choice. Because it's not mainly about fear, but deciding to believe her instincts or ignoring them.
Myriam has never lacked courage, just self belief. And this game is all about her exploring her beliefs and fantasies, and accepting that they're equally as valid as anyone elses. Because Anthony has progressively convinced her, that her opinion doesn't matter and that she's too dumb to work stuff out for herself. This is the nature of abuse. Seemingly intelligent people reduced to disregarding what most people would resist. Abuse is not just about fear, but manipulating people into a mindset where they'll never stand up for themselves, where they'll always accept another interpretation of events over their own.
The fact is, when Myriam actually stands up to Anthony, he always backs down. Because essentially he's a coward, albeit a very manipulative one. Physically Myriam is more than capable of resisting Anthony. She's much taller than him, and probably equally as strong. The big difference between them, is he believes he's always right, whilst she undervalues her own opinions. And Anthony has recognised this, and over time he's convinced her that she's right to undervalue her opinions. And this allows him to manipulate her whenever he wants. This is why Anthony chose her as a partner, because he desperately needs someone to feel superior to. And because Myriam is essentially rich and powerful, her being so controllable, makes her all the more alluring for him
