Hi, Mary. I stumbled across your thread. Welcome to the boards. I'm sorry you have to be here, but you are in good company. Hopefully you will be off moderation soon and some of the veterans (who have WAY better advice than I do!) will come to your rescue. In the meantime, I wanted to give you my support.
I'm so very sorry for what's happened. Is it too late to save your relationship? Of course not! We all make mistakes in trying to get our SO/spouses back. Just recognize what isn't working for you and start trying some of the DB techniques you've read about.
You've made it clear that you are aware of the changes you need to make to improve your relationship. Now it's time to make them! You know what you need to do first...stop the crying, begging, sulking, snooping, etc. Try to focus on keeping yourself healthy. No one wants to be with a red/swollen eyed zombie! Plus, you have that precious child to take care of. So, yes, I know you are at what seems like rock bottom. I know the pain, the devastation, the fear, the anger, the feeling of your heart LITERALLY breaking in your chest, can't sleep, can't eat, etc. But you've got to do your best to embrace these feelings in a positive way so you can work through them. And you need to find some happiness for you right now. Whatever you enjoy doing for you, DO IT.
Next, you need to set some boundaries. It's just downright disrespectful that your SO is texting the OP in front of you! I would calmly and plainly tell her that her communicating with the OP in front of you is hurtful and disrespectful and to please no longer do it. You need to decide what other boundaries you can set to protect your feelings as much as possible.