At the moment she has the stable home life and the teenage, no responsibility relationship.
Read about the 180.
People rewrite history when having an affair, it goes with the territory and allows them to live with the cognitive dissonance an affair creates.
Affairs have a 3% chance of working, she is likely to then come crawling back. But her behaviour is killing your relationship dead. I’d stop being so nice. I don’t mean be nasty I just mean give her consequences. So you want to split, talk to me about how this is going to work? If she deflects back say ‘you’ve had far more time to plan, so what’s the plan?’.
Affairs are fantasy land - a cocktail of chemicals. I read once that an affair lives by burning up everything in your life, your marriage, relationships, self esteem, self worth, your financial security, previous dreams, retirement, then when you have given it all up, used it as fuel to prove the affair is what you need in life, you are wonderful or your marriage was awful or your affair partner is your true soulmate, the affair dies (it was never going to work) leaving you surrounded by ashes. The author wrote it far better by the way.
Sadly I have seen this, when people are left financially worse off, in a smaller leas stable home, with 50/50 shared care of the kids and then the new relationship fails too. They hobble on with the affair longer than they would as they cannot admit they destroyed several peoples lives for fuck all and the soulmate was nothing of the sort. Of course at this stage they all think they have the 3% affair that will be a success 😂.
Chrck out the surviving infidelity website and affair recovery. The former has advice for you even if you are separating. The latter will help you understand what has happened and why it’s not your fault.
I am sorry this has happened and I hope you get the best outcome.