OP, my situation is very similar, but I am a year ahead of you.
You now have two relationships to get over. Get a calendar, and mark out the one year point. That is how long it will take you to feel any kind of happiness again. I wish someone had told me that, so I was more patient with myself. Your karma is that you will cry every day for most of a year, and wake up feeling like you just had the most terrible nightmare, before realising it is your life, constructed by all of your own decisions. Confide in a close friend, get support, you are going to need it to pull through. Spend quality time with your child, they grow up so fast.
Start painting classes, photography, yoga, do anything that is just for you to start rebuilding who you are, as right now you don't really know. And in this year, do not see anyone else, you are not ready. You are attractive and will get further offers of eternal happiness. Say no, rebuild yourself, focus on your diet and fitness, more offers will come when the timing is right. Trust me, that over time, you will see it positively as an exit from your unhappy marriage and exciting when that second chance does come around. Spend all your energy on yourself, it will make you stronger as you begin to value yourself once again. You will develop healthier boundaries, and very importantly you will learn to source happiness from places other than a man. I didn't think I needed to do any of this stuff, but it's the right path to healing and becoming a better, stronger person in the future. You'll never want a married man again.
I wouldn't bother telling her, he will never forgive you, and she will not leave him, not if she is also in her 40's with children and if he provides her with a comfortable lifestyle. She will fear harming her family, and her loss of social status. In my case the wife already knew, and it wasn't the first time, he got caught using paid services some years ago (one reason why their marriage was stuffed, and why I wasn't too wracked with guilt). After he left, she felt her life was ruined and hated being alone so much that when he came back, hysterical bonding kicked in, and she loved him more than ever before. I too felt he got off scot free. Cheaters and narcs are good at identifying weak, naiive women like us. But the nightmares I wrote about above, you will just give them to her as well, and you need to see that she is actually the victim in all of this.
I know how it felt, they way he looked at you, the perfect chemistry, what he said, it was all so emotional and real, and he was probably a black and white kind of guy that you could totally trust. In my case we were in the happiest bubble, even living together, before I was dumped by text message out of the blue while at work one day (classic discard). He went back to his wife, but it wasn't me vs the wife, it was me vs the family. A battle rarely won, and as painful as it is, this is usually the right outcome. Maybe your situation is similar. Rest assured he will never forget you. He will miss you, wonder what life would have been like if he had chosen you, and keep the memories of the time he spent with you. But everything is as it should be now, you have to let go, and find the strength to rise above. Do it gracefully, and be proud of yourself for that.
Still today I am not over the love of my life. I struggle to accept that it wasn't real, and that I never got a proper chance to show him a life with me. I loved him far more than she did, but their history was worth more. Hopefully over time, the memories will fade. But I am at peace now, if that makes sense. You will be too.