I'm about half way through your story. I'm sorry that you have found yourself here.
My deceased WH was also a serial cheater (men can be cheaters) too.
I decided to stay with him.
Although he told me that he cleaned his act up, he never did, all the way up to his death this past March. I never believed him but wanted to. My gut was always right on.
I stayed and endured the trauma. Felt it was my best option. Very confusing time for me living with him. Very traumatic experience. It definitely caused me PTSD, mind movies, meltdowns. Was constantly gaslighted, manipulated and lied to. Never got the full truth of who he was...
When he was flirting with other women, I started to begin to accept it and to suppress my emotions, looked the other way while he continued to emotionally abuse me.
I felt powerless while he did this. And he was so jealous because he said other men were noticing me. I didn't notice, wasn't paying attention if they were. Anyways, this has been my experience dealing with a serial cheater. He also was an alcoholic. And when a person is an addict, one addiction leads to another addiction, leads to another addiction because they are addicts. Maybe your WW is an addict.
It's your call on how you choose to live your life from this day forward. And also understand that your WW may continue this behavior for the rest of your lives together. And who knows, maybe pick up new addictions along the way, it's crazy making. I know that my WH had three addictions to my current knkwledge.
My thoughts are if you do plan to continue the marriage, you will need to become very strong in yourself and like another poster stated, set the bar very high.
Set your demands for what you expect from her and let her do the work. Step way back and do the 180. Your actions (or inactions) will allow her to see the pain that she caused to you.
When I say inactions, quit doing certain things for her that you used to do. She needs to feel the pain she caused to you and your kid's. Use firm discipline like you would with a child. How old are you and how old is your WW? I don't remember seeing your ages.
Since my deceased WH has passed away, I do feel some sort of peace in my heart, I don't have to deal with his emotionally abusive behavior anymore and it is such a relief. I do miss him though despite having to deal his abusive dark side when it reared its ugly head.
I am now learning to relax and be happy and content in my life again after dealing with so much traumatic emotional abuse at the hands of my WH. This is not the outcome I wanted though. I wanted him to fix himself. He either couldn't or wouldn't because the thrill of other women was so much greater than the desire to fix himself and become that awesome husband I wanted and deserved.
I can even say that I still love him despite his dark side, he did have a very loving and generous side to him... I'm just not sure how worth it it was to continue to stay in the marriage with him (for my own mental health).
I do believe I made the right decision to stay for quite a few reasons; for my kid's, because I financially depended on him, because I loved the good side of him, because I married him and felt that marriage was supposed to be forever... messed up and twisted thinking, I know. In the end, I feel I made the right choice to stay for even deeper reasons that I don't want to get into now.
But what is even more messed up is when we feel we have to defend ourselves against the one whom we thought we could love and trust with our lives, the one we each made promises to. And with that, we committed ourselves to each other in faithfulness, love, trust, etc. to and in front of God.
Take your time with your decision on whether to R or D, could be months (or even years) before a decision has to be made. There are so many things you can do before you get to the point of decisionmaking; spend time away from home doing something you enjoy, do something with the kid's, limit contact with her. Let her do the nice things for herself that you would normally do for her, such as making her coffee, washing her car for her, buying her clothes, flowers, jewelery, taking her out on dates, trips, candy, etc.
Show her how it feels for you not to be a part of her life without leaving the family home. Treat her with the indifference, like she did to you. You can also do the soft 180 because I understand that you still need to keep the balance in your home because of the kid's. I do understand.
It's a long journey ( maybe even years before you may feel somewhat normal and safe and it sucks) that you are about to endure and in one way or another, you will get through this. I know because I and the others who here on SI are living proof.
I hope my story can help you in some way. Again, I am sorry you are going through this too. And please remember that we are here for you.