And it was really smooth of her to cut the deal in "marriage counseling." She lured you into an environment in which you expect to work on the marriage and then told you that she wants all of the benefits of marriage--financial security, uninterrupted time with your child, etc.--without any of the work or accountability. Since you'll have a "roommate" relationship, now she will want you to respect her privacy...giving her the freedom to continue her affair without your having any right to know.
Also, a true "roommate" relationship would require either a 50/50 split...if she has the run of the house...or boundaries on what she can do and where she can go in the house. Are you going to establish those boundaries? Since you're paying 3/4 of the expenses of the household, you should have 3/4 of the decision-making power. If you're a married couple it's different, but if you're roommates she should treat you as a landlord and you should treat her as a tennant--you are not a team.
Sorry to be so negative, but I think you just got waylaid.
