It might very well be that the daughter who is having an affair has suspected all along that your reaction would be as strong as it is now: disappointment, hurt, and, well, a strongly worded negative judgment. Further, if the daughter knows you well, she might have suspected that you would feel that her affair reflects negatively on your parenting skills. Clearly, for you, it does, and you then transfer the relationship between you and the daughter onto the relationship between her and your granddaughter, faulting your daughter for setting a bad example, even though the 4-year-old is probably too young to follow in her mother's footsteps, esp. if the affair is concealed in the household (it is not clear whether it is, because her colleagues and manager are, apparently, in the know).

Jeff ^ wrote well: "For whatever reason, your daughter has chosen not to share the details of her relationship with you." But could it be that the reasons the daughter did not confide in you were 1) the expectation of a harsh judgment coming from you, and 2) the desire to spare you the negative emotions from feeling that you failed as a mother in raising a daughter who strays in her marriage?

The affair is clearly long-playing, since it started before your 4-y-o granddaughter was born. Possibly, your son-in-law knows about it, and he and his wife might have entered into a tacit don't ask, don't tell arrangement. Who knows. Maybe he has seen signs but chosen to ignore them to avoid feeling psychic pain. There is no way to determine what is going on in their family. And there is nothing you can do at the moment, so why not step back and read some fiction books or watch some movies on the topic, i.e., where women having affairs are portrayed, to gain some perspective and get out of your current pain in which you probably feel that you are alone with this pain. There are also self-compassion meditations by Kristin Neff where she talks about common humanity– those might help you. Realizing that many grandmothers go through this, finding out their daughters are having affairs, and that realization will make you feel less alone with your pain.

To the extent that your daughter did not share with you because she wanted to protect you from the psychic pain of feeling disappointed and lacking in parenting skills, this shows care and concern on the part of the daughter for the well-being of her mother, which is laudable.

To the extent that your daughter did not share with you because she was trying to avoid being judged harshly by you, this is understandable.

The language you used, in describing your communications with the 35-y-o daughter, sounds a little off: "coaching her." She is too old to be coached by you (Jeff ^ put it better than I can). You advised therapy, and she accepted your advice; this is already very good. It means she values your recommendations. She respects you, and your opinion is highly valuable in her eyes. This is good, but it is also good enough–wanting to coach her is excessive. She will figure out what to do. She is a big girl. Maybe she has been actively discussing the situation with her therapist, who, if said therapist is a consummate professional, provides a confidential and non-judgmental environment. Leave it to them–the daughter, the therapist, possibly her men (the husband and the man she is involved with) to figure out what to do. There is no good scenario with your involvement in their lives given the situation and given your beliefs, so step back and focus on yourself, your own relationships, friendship, physical and mental health, reading, hobbies, travel, fitness, work, volunteering... whatever is most important to you Since you came to this forum, see the Relationships and Communications subforum, which, incidentally, is visited more often that this sub-forum, and read the stories. You will quickly see that there are many stories that describe circumstances much worse than what you have just described. This might provide solace. Better yet, start responding, as you might help other people in the safety of an anonymous setting, and your energy will be redirected from feeling internal pain and beating yourself up over what you believe to be your failure as a parent onto something more constructive and worthwhile.