I am so sorry you are feeling so overwhelmed with the thoughts about what is going to happen when your daughter finds out...can I suggest a few things and I am not sure what you think but firstly your daughter knows you very very well and also is an adult, she will see that you have made a mistake, she will see that you have done everything that you can to make amends for your actions and that you are both very sorry and very remorseful for what you have done...but this is not who you are as a person, this one mistake does not define the rest of your life or who you are as a person, you are still her mother whom she loves. I also want to say that although you are her mother, your mistakes are not her mistakes, just as her mistakes are not yours. You can be there to support each other but usually people are quite reasonable and understand that this really has nothing to do with her and therefore should have no recourse on her. I hear how panicked you are by this but she is an adult and will understand that you have made a mistake, I think I may have mentioned this before too, I think it would be great if you are able to be the one to tell her, I think she will receive this information if she feels like you can show her the place she has in your life and that you want to let her know what you have done. It would be hard I would think for her to hear it from someone else and wonder why you didn't tell her.
I hope that you are able to get a hug off your parents today and take a moment to breathe and be kind to you. You have so very much going on and I am glad you are in a place of love and support.
Hugs to you LQ23