Thank you for the additional information. It's normal to feel mixed feelings about the fact that he now has a girlfriend. Especially when you two are newly separated and though it's been a bumpy ride, it's still a huge decision to split, you have a son together, etc.
It does sound like you have realized many issues that were causing tension for you two, and that you also were disconnected. It's also very possible that if he didn't have a girlfriend right now you would be feeling more at peace with the split, but this creates a new dynamic for you, where you may be feeling hurt, angry and maybe even some jealousy...this is all normal. And these feelings don't indicate however, that you want to be back with him, though it's possible that if you two resolved some of these previous issues things could be better.
That is the key.
Things will not be better in the marriage, unless you both really address what is missing or what was wrong and change those things. It's hard work and often doesn't work out... and sometimes does of course.
For now, it would be wise to continue focusing on yourself, being social, seeing friends etc. and also focusing on creating stability for your son and lots of attention and reassurance towards your son as well. Dating can always come later when things are finalized and everyone is stable and grounded.
Try to focus on positive things and dont dwell on the fact that he has a girlfriend already. Most of these quick relationships while separated tend to be about rebounding or about dependence and quickly fizzle out... he would be better served to work on himself, and get clear before jumping into another relationship. But that is something that you can do for yourself, and will be better off in the longterm for both you and your son.
You may want to consider some counseling right now just to get support during this transitional time. If you would like to provide your zip code I will send you some therapists in your area. Please click ACCEPT button for this answer and feel free to continue discussion even after clicking accept. Thank you and best wishes.