Your husband's delusions, revealed during mediation, have spurred me to my first ever post on Mumsnet! You must not allow his ridiculous assertions that she is so supportive and wonderful whilst you were not to get to you as they are based on a fantasy!
OF COURSE she is being everything he ever dreamed of right now - she is still competing for him and knows she needs to secure outright victory before allowing any hint of reality into their relationship. She is currently projecting the most idealised version of herself possible to make sure she keeps him. So it'll be all, 'you're amazing, you're a brilliant dad, you can do anything you want, you are never wrong,' which is almost as addictive to idiots like him as blow jobs in stationery cupboards.
This sort of heightened regard for each other is normal in the first flush of any new romantic relationship but is particularly strong here because she is working hard to get her 'prize'.
So he thinks she's the bees' knees because she is constantly and expertly massaging his ego. Compared to the real, everyday, ordinary family life (which I think is pretty awesome actually) that he's had with you, it's easy for men like him to compare and have you come up short. Arse.
But, as in any relationship, it can't last, she will not be able to keep it up. Ultimately the hoovering needs doing and bills need paying and the children get ill. Suddenly it's not all trips to the pub after work, pathetic selfies on Facebook, parading her at weddings and parties (in front of people who are actually embarrassed for them), 'ooh, you're so amazing!', 'ooh you are too kissy, kissy,' - suddenly it's REAL LIFE. The grass isn't greener. She's not giving him on-demand blow jobs. She's not telling him how fab and wonderful he is. Then he starts to think, 'shit, what have I done?! My lovely wife, all our history is destroyed. My poor children, what have I done to them. Our beautiful home- I've nowhere to live except this lonely flat. Everyone at work thinks I'm a tit. Etc. etc.'
But Phee I PROMISE - you will be so far over him by then. You will have the children, the house, a new job, perhaps even a new partner. He will be gutted when it hits him. He will realise how selfish and pathetic he's been and for what? A fantasy. Well ha bloody ha on him.
I am certain their relationship won't last because it is not based on reality. If he'd sat there in mediation and said, 'WF and I have our ups and downs and we're struggling with what we've done so we're going to live separately for as long as needed and take our time before making anything more permanent out of respect for Phee and the sake of our children,' or something like that you'd think he might have gone into it eyes wide open and they might have a chance. But everything he says and does indicates it's all one big dream fairy tale bubble that will pop at some point.
Keep going as you are; soon it will feel better, I promise.