You have gotten some good advice and support so far. I think you are on the right track finding out as much as you can and staying on top of what is going on for you. I hope you and he are doing the searching and learning together. Perhaps he would like to chime in at some point.
there is a lot on here about cheating and affairs. If you do a search for tags and look at the stickies you will find a lot of information about what happens for people in their marriages and beyond when an affair happens. I hate to say it but I agree with SJ, not a good outcome usually and he hasn't had a could start with the healing process it seems.
As to your husband. He is being selfish and uncaring. He has not empathized with you and what you might be feeling and has not considered anyone but himself. Not even this woman that he has this energy with. She is in this too and he is assuming that he can just paste the two of you together. I have spent years of work getting to where I am in poly relationships... it just doesn't happen like that for intelligent, emotional animals such as humans.
the only think at this point that he has going for him is that he has finally been honest. Well, he has 9 months to make up for that. that trust you had in him is likely gone forever and he is worried about losing this woman?! Wow.... wake up call, he could very well lose you if he doesn't get busy and start doing the work necessary to begin healing.
I agree, you could use this to make your relationship happy and healthy again, but I think that he needs to back right up to a time before he decided to cross the line and be at arms length from her for a time until you are comfortable, if ever. that to me should be the result of his actions and what I would suggest. His face and heart having been going in her direction, a firm set of boundaries might just bring him around... as much as he will hate that
As far as I'm concerned if he wants your trust he will go with whatever boundaries you have and do it willingly and humbly.
I know I sound firm in this, but really, having read peoples stories on here for 18 months and experienced my own stuff, I think he should not be encouraged to lay around in that "energy" one moment longer and should get back on track with fixing what he and she have broken. Maybe, if he is lucky, he will be able to have her in his life also...
You have every right to be angry, hurt, broken-hearted and devastated. He has broken a huge commitment and if that weren't bad enough, he is trying to rationalize it and make it out to be a thing of beauty. What would of been a thing of beauty would of been bonding with you over being honest about his feelings when you asked him. He didn't and now he has lost his integrity by being dishonest, disrespectful, and not communicating.... the three foundations of what poly is about; honesty, communication and respect.... how about he get on board with that with you so that you can trust him again.
Trust is not a given or a right. It is earned and he lost the right to your trust now... it is not a given that you will just move over for this woman. She has to earn your trust also, she cheated with him. Who cares about the "energy" that is all lovely, it has nothing to do with you and isn't based in reality.... the reality is that they fucked up by giving in to it for nine months.... now it is bad energy if you ask me... and will take you going at the pace that you want to make it good again...
I'm sorry he is hurt and having a hard time, but it's not his time, he had that.... or rather took it. it's time to get back on track with his commitment to you... that is my thoughts.