I am speaking to you as someone who was a former disloyal spouse, so I've BTDT and have the ability to speak to you maybe as a triggered loyal spouse can not. And one thing stand out to me like a flashing, glaring red light: you do not take personal responsibility for ADULTERY. Oh you don't say it was someone else's fault or say it was the "right" thing to do, but you act like you are not entirely responsible "because you have an addictive personality" or "you have an addiction". In addition, rather than naming it what it is, you skirt around with euphemisms and minimizing phrases like you were "flirting" on FB or "getting an ego boost" or whatever terms you have in your head.
So #1 I would highly encourage you to stop minimizing what you did. You are not some naive, innocent little waif who went tripping into a minefield over which you have no control and blew off your leg. You did not harmlessly flirt. You did not enjoy "just a fantasy." It was ADULTERY. Say that out loud and embrace the fact that something inside you balanced "the safety of the people I love" vs. "the thrill of adultery" and picked ADULTERY as the winner! Here's why that's important: until you can stop minimizing what you've done, you will spend a lot of time trying to convince others it wasn't all that bad, it didn't go that far, you shouldn't hurt that much, etc. In reality you made a promise to your husband to forsake ALL OTHERS (all...not most, or some). In reality you made a promise to spend your life learning about him and behaving in a loving way toward him (not "staying if I get my needs met" or "... if you make me feel XYZ"). Thus, you are the character of person --right now-- who not only breaks their promises, but also blames others and tries to hurt them in order to feel better about yourself! Now I don't say this necessarily to hurt you or call names, but rather to speak truth and hopefully to shock you that you have sunk that low! Because once you are honest with yourself about how love you have sunk, then you can begin to be honest with yourself and start to do the work to be a better person. As long as you're still lying to yourself "it was just a fantasy" it makes real change impossible.
#2 I would highly encourage you to stop minimizing your responsibility. You purposely chose to commit emotional adultery over and over, even knowing that the price of doing so would be losing your family. No one forced you. No one tricked you. You are not the victim. You KNEW that acting like this would do irreparable harm to the man you claim to supposedly love and the father of your child, and you KNEW that the result would be tearing up your child's security and home...and you made the conscious choice to pursue it anyway. You are not helpless. Your addictive personality is not like a flu that "has to" run it's course. That puts the control in the addiction's hands rather than where it firmly belongs: YOUR HANDS. The men on FB are not responsible. Your husband not giving you as big of a thrill is not responsible. Your addictive habits are not responsible. You are an adult and you CAN be self-aware enough to know you have a weakness, and you can choose to do something to protect your family from your own weakness, but you made the conscious decision to do NOTHING> So, stop hiding this from yourself, and you may be able to take a step forward.
Finally I would STRONGLY encourage you to be upright and honest with your little girl. Her world is about to be blown apart and she is going to lose half her time with her mom, and half her time with her dad, and she deserves a REAL answer! She deserves the truth. And yes it's going to hurt her, but you chose to do this, so now you are experiencing the natural consequence of your choice. See, you are not stupid. You're a mental health professional! You know that each and every choice we make--in public and in private--has both a BENEFIT and a COST. The benefit of your fantasies was that you got the thrill. You felt interesting and attractive and maybe "loved.' But the cost of your fantasies is the look in your little girl's eyes when she hears that her and her daddy are moving out because you picked fantasies over her safety! And yes, don't kid yourself--you did!!
If you want to make it right, the way to do that is not to keep lying!! The way to make is right is to stop being dishonest, experience the consequences of what you chose to do, work on yourself to become a better woman-mother-wife, and continue to be honest even when it's scary or hurts. THIS is how people might gradually begin to trust you again, because right now you have demonstrated to them, in your actions, that you are not trustworthy! And even though it is going to HURT YOU when you tell your little girl, you can tel her honestly in an age appropriate way why her daddy is not coming. I liked this suggestion earlier:
"Mummy has been doing some bad things & I've hurt Daddy. He needs some grown-up time away from me to think about things. You are the most important thing in his life. He's very sad that he will miss spending Christmas Eve with you. He will be thinking about you all the time as he always does. Both Mummy & Daddy love you very much. If you have any questions,ever, you can ask either of us. We can take lots of photographs for you to show Daddy. He will really enjoy sharing that with you."
Try working with that and making it sound like something you would say. She needs to understand that YOU did something hurtful, that her dad is not being a jerk or blaming you, that he is not "at fault" and has a right to be hurt, and that it is reasonable for someone to take some time to figure things out if they were hurt. This will be a lesson to her--people don't just break up over "not getting along" or "drifting apart" (otherwise what if she "doesn't get along" with a teacher at school--can she break up with that teacher?). She can be taught that everyone, even mom and dad, do something wrong, and that the way to FIX IT when you've done something wrong, is to admit exactly what you've done, take responsibility for it, ask for forgiveness, and then STOP DOING the wrong thing.