First off, whether you made mistakes or not.. Her choice to be adulterous is her choice. It is never the victim's fault. They always have the choice to get out of the existing relationship before making a new one, regardless of the reasons, it's all on them if they choose to have it both ways. Period.

I won't even begin to ask you about what you "did wrong". What I will say is her "yes" not being what you expected should be more than a disappointment. It's a huge warning sign even without the additional relationship. She is already telling you to change before you've even tied the knot. Not that it's ok after you do but that it shows how your relationship is going to go into the future. Everyone has shortfalls in life and when one person is busy looking at the other person and what they need to change, they tend to miss their own shortfalls. So even before the adulterous acts happened, I would have a the very least said "hold off" on even consideirng marriage.

As for the rest of it, it may have been unethical to spy on your SO, but you had reasons, but I'm not so sure I'd place your "wrong" at a level higher than outright cheating on you. She has no place to cast the first stone at you for doing what she was doign behind your back. So meh.. It is probably best you found out now. Cheaters tend to keep doing what they are doing until they are called on their behaviors so who knows how long it would go on.

it is up to you at this point, what you do, but from my perspective, I would personally move on. Since she has lied again about ending the other relationship and continued, she clearly shows she has no intention of changing this. Therefore I see no reason for you to pursue her further.

I know this may not be easy to hear but I feel for you and I think you deserve better.