You've truly been an inspiration and have gone waaaay beyond what most people would do to try and save a marriage. Don't regret trying, and don't regret wanting to believe him and giving him those chances. You bestowed great love, and respect on your husband and your family. You also gave him plenty of chances to appreciate you and having his family together. But it sounds like he really wants something else and that's just the reality. The good thing is there's no need for any more pretending or lies. Just let him go. I know you are angry and mad. But let that go as well. You don't have time for it and you need to focus 100% on YOU. Treat yourself very very well. Especially now during the holidays. You know the reality and it's now time to accept it and live it. Before you file, emotionally LET GO.
Start working on you and your future... and a future without him. Get yourself emotionally and financially in a good place. Focus more on your kids, friends, family and other support systems. Make your goal a healthy self-love, self-esteem and letting go of any anger, wishing husband and OW the best (even though you'd like to smash strawberry whipped cream pies in their faces), and go CREATE A GREAT LIFE FOR YOURSELF. Cry and accept the relationship with your husband is over, go through that pain, but then see the opportunity. You have a chance to start new, make a terrific life for yourself, get involved in some positive things, spend more time with friends and family.... Yes, you would have preferred having your family together, but that's just not the reality. Time to create a new one. No more pretending, no more lies.
Also, keep in mind, there's no reason to rush into a divorce. I'm not saying that to try and save your marriage. I just think it's wise to take a little time and get your ducks financially in order prior to filing.
Except, well, there is one reason you might want to stay legally "married".... If you want to retain more control over who your kids are with. But if you decide to do this you must do so with eyes fully open (knowing there is OW or women) and accepting it's an "open marriage." You are just parental partners, and you and your husband lead separate lives. No expectations of each other.... and then file when your youngest turns 17. But that's a big sacrifice. Weigh everything carefully and do what is best for YOU.
{{{{{{{{hugs}}}}}}}}}}
There is no arriving, ever. It is all a continual becoming.