I can’t diagnose her on the basis of a letter. I can highlight for you that you have spent 2 years trying to be patient and to give her the room to come to her senses. She hasn’t taken you up on it. While you have been raising your son, she has been “finding herself” and apparently finding other men. She says she doesn’t like what she is doing but she doesn’t stop it — nor does she seek treatment to help her figure out what she is doing to herself and to you and her son. Please ask yourself if this is what you want your son to grow up believing is a normal and healthy relationship with someone he loves.
You can’t make her change. What you can do is change your response to what she is doing. Insist that she go with you to couples therapy and perhaps get some individual treatment as well. If she won’t, you have a hard choice to make. You can either keep on letting her set the terms for your marriage or you can make your next stop be at a lawyer’s office to file for divorce. Only you know which feels like it will be right over the long run. For what it’s worth: I do think you and your son deserve better.
I wish you well.