So, the only thing he didn't know about was an incident of oral sex sixteen years ago before marriage, and he's saying that he wouldn't have married you or had children with you if he'd known about it. You didn't actually get married until something like seven years later, right? And this was the only thing he didn't know at the time he agreed to the marriage?
Let me share something with you... the prefrontal cortex of the brain doesn't fully mature until we're about 25 years old. That's the judgment center. So, you're being held to account for something which happened before your brain was fully matured. His argument... well, you hid it. Yes you did. When you made the initial decision to hide it, your brain wasn't fully matured. But later, and before the marriage, you might have reviewed that earlier piece of information and you didn't. THAT is the ONLY thing you're guilty of. And as mitigating facts, you've been a good and faithful wife and mother for NINE years.
You know what? I had sex before marriage. Lots of sex before marriage. I had boyfriends and hook-ups, and yes... there was some oral in there too. And you know what my WH knows about all that?... fuck all. It's not his business. We weren't married yet. I took a vow of fidelity when I married him and I have held to it. And even though I am the promiscuous one, it was him who cheated. But... I can hear the argument... you were exclusive sixteen years ago and he didn't know about the blow job. He wouldn't have married you if he'd known. And my answer is... that he doesn't love you and he never did. Further, you might not have married a man so prudish and judgmental if you had known. So, you BOTH didn't know something. He's not the only one who got sold a bill of goods. A man who loves you might be upset, let down, etc. He might feel a bit cheated that he wasn't getting blow jobs earlier in the marriage or whatever. But your fifteen years together would mitigate his irrational urge to punish you for something you did when you were essentially a child and BEFORE marriage.
I don't believe he's being honest with you. I think he's keeping you sweet while he divorces you and that his promise to reconcile is worthless. Even if he did allow R, he's demoting you from wife to girlfriend with no promise of remarriage. That's putting you in the "one-down" in perpetuity. A lifetime punishment, as it were. Frankly, you still sound like you're young enough to be a wife and maybe even have a few more children. Why would you sacrifice your status as a wife and mother on the off-chance of forgiveness by a guy who's FIRST inclination is not to consider how much you've been through together or whether you've remediated the internal flaws which allowed you to lie all those many years ago, but instead, wants to put you in a permanent one-down? You ask "How can I show him I've changed?"
But he should be able to SEE that in the nine years you've spent together as man and wife. Those years should have meant something to him.
I wasn't expecting my thread to go this way. I feel a very strong need to defend my BH and to continue to try to see things his way. He is a hurt soul right now and I want to help him heal. I truly love this man and the family we have built together.
For typical BS healing, the schedule is something like two to five years. That's even when very bad things have happened, not just before the marriage but during, and not just one incident, but many. Healing also requires the BS to be a bit proactive in seeking out therapy and healing solutions. So how about this?.... before you agree to throwing out the baby along with the bathwater, why not try some traditional therapies? IC for both of you and MC together. You would do well to work with a therapist who is well-qualified in infidelity so that s/he can empathize with your WH's feelings. It's only been eleven days since he found out this missing piece of information, right? Doesn't it make more sense that before he throws away his entire family dynamic that he give traditional therapy a real shot. Then, if he can't find healing in five years... go ahead and give him a divorce on good terms, but NOT custody of the children. Kids NEED both parents. They're innocent. They should have their truest needs met regardless of what's going on between you and their father. If that won't satisfy him, than I think it's a fair conclusion that NOTHING was going to and that he's tricking you into an unfair financial settlement by plying your guilt.
At that point, you should lawyer up and fight back for what's fair.
Don't trust someone who is telling you with their actions that they don't love you. Sweetie, you couldn't GET more hurt than I was after my own DDay, and hell yeah, I was bent on divorce that first week. But, I calmed down and I realized that my fWH was willing to do whatever it took to prove himself, and because I loved him, I gave him that chance.
I always say that no cheater is OWED a second chance, and I mean that. If your WH feels like nothing but divorce will make it right, fine. But that doesn't mean he should be allowed to cheat you out of what you're owed. The majority of courts won't agree to it anyway. So if he doesn't back down on this, you should fight back for your children and a fair settlement. Remember that love isn't enough to keep a marriage going. There has to be respect as well. You don't get respect by laying your neck on the execution block. You get it by being a fully matured woman who can (and will) manage her business. So, if your WH is looking for "sorry"?.. yeah, you're sorry. You were young and stupid and your brain wasn't fully matured. If he's looking for "change"?.. you've proved your change in nine faithful years of marriage. If he's looking to fuck you out of custody of your children and your half of the marital assets?... NO. Get an attorney and fight for what's yours. Don't think that you're losing anything either, because if it comes to that, you can rest assured that he didn't really care for you.