P17

I am in a situation similar to yours. My WS is currently living down the street from me and I do pass that house in order to go home. I am forced to get communications from her because we share property and pets, even though the communication annoys me. I miss my wife like all hell but the advice you are being given here is good advice.
Here is what you are going through. Right now you are going through the bargaining stage of grief. You would do whatever you could to have your wife and family back together. You feel you have lost your wife, and want her back, therefore you are in a generous and giving mood because your priority is her. This has to end. Believe me, no contact works. It really, really does. It makes you feel stronger and better about yourself. It also has a better chance of drawing the other person back towards you (but that is not the goal). The goal is to give you your strength back by not having her around to remind you of the loss. Remember the phrase "Out of sight, out of mind." So true.
You are fortunate that all those strong feelings you have can be transferred to someone else, your child. So take all those warm feelings, and the feeling of being generous or loved, and do things for/with your child. This will help mitigate the damage HER actions (not yours) have caused your child.
Now, you mentioned that you feared that having no contact wouuld make her less interested or concerned about you. It doesn't and here's why. You noticed it when she removed herself from your life. If you do not contact her, then the same will happen to her. And yes, she does care, cause if she didn't, you would be divorced now. When people engage in these behaviors, they are seeking an instant gratification. This is an act based on desperation rather than logic. This will continue as long as they are in pain. If the guy she is seeing has all of these negative qualities, it will not last (they rarely do). But she is unable to stop now because she's desperate to stop the pain. By removing yourself from her life, you are leaving a void that cannot be filled by him, no matter what you might fear. So there's a better chance that she will be drawn back to you to fill the void again.
So to sum up, the first step is to stay positive. She has betrayed you and yet you still love and cherish her enough to forgive. There's a saying that "forgiveness is the ultimate sign of strength." By surviving, even tenuously, you are stronger than she is at this point, since she acting out of a sense of desperation. People are drawn to strength!
Second, maintain as little contact with her as possible. Do NOT let her hang out at the house with your child. She can bond with her at her own house. Plus that'll give her a taste of what it would be like to stay with this guy permanently. I'm sure that is not appealling to either of them. Do not tell her much about your life and keep conversations with her short. None of that is her business.
Third, take care of yourself at all times. Be extremely selfish and indulge yourself. Any non-selfish impulses should be directed to your child.
Fourth, if you feel yourself slipping on the no contact, remember how bad she made you feel and feed a little bit off that anger. That is a primitive but sometimes effective method of keeping yourself focused.
Last, make a plan and stick to it. Decide how long you are willing to wait on her and make plans if it goes either way.

I promise you that everything will be OK. If you are not in jail for homicide or in the hospital on suicide watch, then you have a great deal of strength in you. She WILL notice it, and others will as well. If you continue to move on with life, you will recover. And sometimes, if you move forward with life, some of the things you left behind will catch up. Good luck.