These things take years to get over. You are just 3 weeks out and still in shock. It’s perfectly normal. You still have love in your heart now mixed with lots of other emotions. You’re not close to ready to try any rebuilding and she’s not yet a safe person who can be a part of a new relationship. The old one may be dead, but it definitely is still warm.
So make her no promises. You were right to call off the wedding. Tell her you will never be with anyone who has someone else in their heart and if she ever figures out what she wants, and if it truly is you, she starts rebuilding but showing you everything. All the texts. Writes a timeline of every interaction between them, including sexual. No secrets. No protecting him and keeping things precious.
If he is the love of her life, who are you to keep her from happiness. She should be with him. But you will not be in her life if that’s the case and I recommend being as NC with her as possible. Total if you can.
All that said. I have to be honest with you. She broke trust. She was with another man intimately for a year. You’re never gonna look at her the same again. At least not for a long long time and probably only if you meet up down the road, years from now and decide to try again after experiencing life with other women for a while when you are ready.
She has a lot of work to do on herself. You can’t do it for her. And it’s not a quick fix. She needs to start therapy with an Infidelity specialist for a long time. I wouldn’t even talk to her about a possible future and how she can make you feel safe again until she’s been in such counseling for at least a year.
Reconciliation is not for the faint of heart. And from what your have written, she is in no way yet a candidate.
I could give you 25 things she should be doing right now. Honestly begging aside, she’s not doing even one. She has a long way to go to figure out what it means, truly means, to be remorseful. And honestly, she’s thinking g only of herself right now and not a bit about the pain she caused you.
I tell all BS’s here, move on. If they say they’ll change, move on, if they promise they’ll never do it again, move on. If they say they’ll do the work, move on. If they even start doing the work (we can discuss what that entails) move on.
Until they’ve ACTUALLY DONE THE WORK, actually put in the sweat and effort to show you what a life with you truly means to them. Fixed themselves in therapy. Opened themselves up to you completely. Found disdain for the AP who helped them hurt you so greatly, the man she claims to love, and so many other things, you move on.
If she is one of the rare few, who can put in years of effort. Only once she’s done that and proved to you she’s committed, then, and only then, do your offer even discussing the possibility of rebuilding a new relationship.
That’s my advice. And from the outside looking in right now, she doesn’t have it in her. So start detaching. Move on.
Finally look at my tag line below. I was in a similar situation as you. A year later, I found the love of my life. I have never regretted moving on from a cheater I knew didn’t have it in her to fix what she did, and didn’t hold me solely in her heart.