Need out of relationship, but don't want to be cold hearted

I am common law married to a woman, we have a 4 year old daughter together, and she has a 9 year old son from a previous marriage. She has cheated on me multiple times in the last 3 years, this has driven us a apart and we are not happy together anymore. She is basically using me at this point because she doesn't have a license, and has no money because she hasn't worked in 3 years. She does have a couple of health problems but even her doctor has told her that they would be alleviated by some regular physical activity. She knows i'm not the type to leave her knowing shes got these problems, and she uses that as her ticket to mess around on me. Since this last round of her cheating on me I have decided it was time to move on with my life, and with perfect timing, an old acquaintance comes back into my life, we both had feelings for each other in High School but couldn't express it. so I am ready to see what could happen between us. So my big dilema is two ply. 1) she is a very unfit mother, she uses her problems as a crutch and a reason not to make any contribution around the house, but she feels fine if she is out shopping, or wants to go to the Y and work out, she sleeps all day long, leaving our 4 year old to fend for herself rummaging in the fridge. I came home one day and she had gotten into a whole jar of skin cream, and had it smeared all over her body and all over the couch and floor. She got herself dressed a couple months ago, and decided to leave and go walk to her nana and papas house (3 blocks away) and that is the tip of the iceberg. I have documentation to prove most of this as well, but to bring to light the things I need to to prove she is unfit, could cause her to lose her son too, I would take him in a heartbeat as well, but I don't have any legal right to him. AND (2) the other part of my dilema is the fact that I cannot support two households, but I am not the cold hearted type to just up leave and leave her without a way to support herself and her son. And her parents know about her lack of effort around the house and wont let her move in with them. I am so ready to move on, but the 9 year old does not deserve to suffer for the way his mom acts. any kind of advise in the situation would be greatly appreciated.

I think you should move out and your wife will just have to start helping around the house at her parents. Also you can gain custody of the 9 year old. You shouldn't keep the 9 year old with your wife like that, that wouldn't be fair to him