Her feelings of betrayal for finally scraping together a bare minimum of decency to be truthful with you are not your concern. That's on her.
Your first concern should be your mental and physical health -- and the wellbeing of your daughters. I understand fully the hesitation to divorce. My DDAY with my WW was nearly 5 years ago, although I had a DDAY 2 a little more than a year and a half ago. I am not divorced. I am not separated. I allowed my WW to trickle truth, blameshifting, foot drag on truth, and push rugsweeping for several years. It's no way to live.
Divorce is a life changing decision for you and your girls. So you shouldn't leap to it, but also pay attention to what your mind and body are telling you. It sounds like this was a dealbreaker, because she compounded her toxic and vile actions with a mountain of lies for years.
You have experienced now a period of great relief, and you and your WW are experiencing a post-disclosure respite. I hope this sticks around for you, but please don't take this as anything other than trying to help: Don't beat yourself up if in another week or two you begin to feel differently. This isn't a linear process, and your feelings will be all over the map.
Be prepared for different emotions to re-surface. It sounds like you may be entering a period of hysterical bonding so pay attention to this process. It can be good, it can be great, it can also be not great.
You might consider reading a thread in the General forum here on SI on what is now termed trauma bonding.
Consider now that you know the details: adulterous acts are a form of abuse. Your WW has been lying to your face for years. Lying and then lying some more. That alone is abuse. She also placed your physical health at substantial risk from STD's. That's willful neglect and abuse. She privileged her own comfort and desires over yours, essentially keeping you in a cage these past many years while she denied the truth. I could go on.
Ask: What would you advise another person to do who has been placed in an unsafe situation of abuse?
You might read:
-How to Help Your Spouse Heal from Your Affair (a short book that will demonstrate to you the contrast between your WW and a remorseful wayward spouse -- perhaps your WW has it in her to do what the book recommends. I hope so.)
-Cheating in a Nutshell (another short book that details the storm now happening in your mind and body -- and how you can think about it and cope with it)
-No More Mr. Nice Guy (another short book that could assist you in spotting areas where you might be falling into "nice guy" feedback loops and thinking traps)
-The Way of the Superior Man (another short book to re-center your own masculinity and purpose, which should be independent of your now tarnished wife)
All of these books can each be read in a day.
I would also set up an appointment with a divorce attorney (consults now usually cost about $500) so that you can better understand the divorce process. Your feeling now is not to divorce. That's good. I can tell you I did this late last year and it made a tremendous difference in how I see things now. It made the abstract tangible and helped me begin planning what a divorce would look like. It also helps you better understand D vs. R and makes your decision making a bit more clear whatever you decide to do.
I would schedule an appointment with a betrayal trauma specialist for individual counseling if you can find one in your area. You have been subjected to dire and severe betrayal trauma, among the worst experiences a human can endure. You need help.
Lastly, you might read about and consider a therapeutic separation of several weeks, perhaps 30 days. Ask her to move out and stay with her parents. If she can't stay faithful during this time, it's a great stress test for exactly how remorseful she is.
This can provide you with perspective about what you want, get a glimpse of life without her, and put you in a place of mental and physical safety where you are not constantly re-triggered by her presence. EDIT: You are leaning toward remaining together, so a therapeutic separation is just something to keep in your pocket as an option for you if you begin to feel differently.
[This message edited by Thumos at 3:57 PM, Friday, September 17th]