If a known, active wayward wants to move out of the house, you can 100% guarantee this is so that they can spend more time with the other person without the guilt of having you around asking them questions.
He's not moving out to "figure things out". That's a bald-faced lie to throw you off the truth. Waywards lie to conceal their relationship. It's all part of the script they follow. He's moving out so he can have sex with her without your interference. His feelings were mixed because he just came off a high of a couple of days of interference-free sex, yet felt incredibly guilty and needed to talk to you to try to let you down easy by saying "I love you but I'm not in love with you."
He'll try to blame his change in feelings on ANYTHING but the affair. He'll try to tell you it's your fault, or that you're imagining things (gaslighting). He'll tell you he's felt this way for a very long time but couldn't talk to you about it.
That's all crap they make up when under the influence of "contrast effect": he has this new relationship, having wild sex, abundant attention, and ZERO time spent solving problems with another woman, and compares it now with his sedate home life, dealing with children and conflict, and the contrast makes things at home look much worse than they really were.
Often, on this board, we refer to "aliens" taking over our spouse's bodies. It's helpful to think of a wayward spouse this way. Now you know that he's going to act completely out of character.
Go take a few hours and read what Dr. Harley has to say about infidelity, then come back and we can help you.
http://www.marriagebuilders.com/graphic/mbi5525_qa.html