She did tell me she regretted doing it

I think it's more the case that she regrets that you found out.

I have tried to talk with her, to the point of going to relationship counseling but it hasnt really helped

The reason that it hasn't helped is because she's not willing to face the truth/reality of her actions/choices. She lies to you about it and shuts down when you want to talk to her about it because she does want to face up to having a 2 year affair with a married man, some of that overlapping with your relationship with her (even if you weren't exclusive), because she doesn't want to think of herself, or you to think of her, as the shitty person that she is. It's not like she didn't know he was married. Counseling is only going to help if you let it and you're honest with yourself and the counselor. She wasn't doing any of that because she'd have to face the reality of what she did and the possible consequences of her actions. Basically, OP, she's being just as selfish now as she was when she was having that 2 year affair and I think that you need to face the fact, knowing what you know, that you can't trust her.