My two cents:
1) she never got over your affair and doesn't believe it was just emotional. You moved out and started dating someone right away from your past whom you probably had sex with at one point. You hear how it sounds, right? Even if you are telling the truth, it sounds fishy. Also, to her, you essentially dumped her for an ex. Ouch! Worse, you felt you could confide in another woman and not her. That's you saying you don't trust your wife with your feelings.
I'm not judging. Just trying to see if a different perspective feels right to you.
2) since then, she doesn't feel like you can trust her with the real truth (remember, she thinks you had to do SOMETHING physical with your ex). She also doesn't feel like you can trust her with your feelings or that she's who you will ever come to in times of need.
She feels like she'd plan B.
So, she sought out what you weren't giving her (I'm not giving a pass, just offering a perspective), and found herself in an affair. The question is, did it get her what she was looking for? And when she can answer that, maybe there's hope. Be prepared that it DID give her what she wanted/needed, and therefore has the self-confidence to move forward with or without you. If it didn't, she should really work on that or she'll be open to more affairs, despite her insistence that she wouldn't (because she truly believe she wouldn't). Or...
3) The affair isn't over and she wants YOU to give up on her... again. Because in her eyes, you gave up on her the first time.
4) She's being totally honest with you in what happened hoping that you'll do that in return (remember, she probably 100% believes you were both romantically and physically intimate with your ex).
5) I'm betting she just wanted to feel important to someone since she feels you are and always have your ex in your head, heart, groin.
Okay, so it wasn't two cents. It was more a nickel's worth. But I'd bet that if you went over some of these with her, she'd agree with some of it.
Want your best shot at working this out? Spin this in a way that shows her you see exactly how she feels. "I know you are hurting. You probably think X, Y, Z, and looking at what I did, I don't blame you for thinking that way."
I'm not saying to give her a pass. She does need to come to terms with this. And she's probably afraid of a therapist because they'll call her out on a malicious revenge affair, and maybe that wasn't really what she was doing. Maybe she truly feels second rate and wanted to feel important to someone else, fantasy or not. It may not have been malicious. Again, just another perspective on this.
Wishing you both healing, whichever way it goes.