*hugs* to you. I'm sorry you're having to deal with all this, but as your thread title says: this is a new beginning for you. Beginnings, like birth, do involve some amount of pain as we make our way into a new life. Try to remember, especially when things get bleak, that no one ever died from being uncomfortable, whether physically or emotionally. It will pass and time will march on, as it does whether we like it or not.
As for your stepson...boy can I ever relate to you. I was married to my XAH for 3 years and was with him for 6. During that period, I did my best to coparent his 6 year old son, who was being shuttled around between his dad's many apartments and his mother's chaotic home. He had no manners and was like a wild animal. I don't want to toot my own horn, but I managed to normalize things for him by steadying his father, at least for a while. I watched that little boy grow up into a pre-teen and navigated the rather scary waters of being "a stepmom". I really love my stepson and I miss him tremendously.
In my case, things got dramatic and XAH decided to forbid me to see his son because he was afraid I'd "turn him" against him. Silliness. This is where I really had to practice detachment, and I struggle with it every day. I wrote to my former stepson, telling him how guilty I felt about leaving him behind, about not being able to save him. I left the lines of communication open for him, but sadly, his father broke into his email and once again threatened me if I didn't stay away. So I've stayed away. It broke my heart, but for the sake of keeping drama to a minimum, I made the choice of no contact. Perhaps once my former stepson is older (he should be an adult in 5 years), he'll want to get in touch with me, if only to get to know his sister, my DD. If not, I hand all this over to HP and trust that my stepson is in good hands. I've already received news that my stepson told his father in no uncertain words that he wanted nothing to do with him, and moved back home to live with his mother. Just that tidbit let me know that HP is watching over him.
In your case, I would make gentle overtures to your stepson's mother, and see what comes of it. Perhaps offer to take her son out every month for a special outing...like dinner + a movie, just the two of you. You don't have to be his stepmom to be involved in his life....