Ophelia, I have been lurking on your thread from the beginning and just want to say I am in awe of you. I went through similar about 18 years ago and floundered for ages. If mumsnet had been about maybe I wouldn't have struggled for so long.
All the posters are saying every bit of advice that I would give you and I say hear hear!
I know you will think I am bonkers but I would genuinely put my house on the fact that he will be hit by reality in a couple of months and will come crawling back begging forgiveness and will invariably use the words 'mid life crisis'.
I am approaching 50 and have seen this scenario so many times. It is utterly predictable behaviour. Nobody on the planet has an illicit affair for a few months in the workplace, for removed from reality, and moves on to another family and lives happily ever after. Trust me they will find themselves immersed in the complication of pretending to be a family. They haven't a hope in hell.
Relationships are extremely hard work and involve an awful lot of compromise which goes on for years and years. Your ex and his new gf are living in cloud cuckoo land. This whole business of her being so reasonable with her ex and your exh trying to emulate it is so false it makes me sick.
It is bullshit and I promise you that it won't last. The only comfort I can give you is that by going through this crisis you will end up knowing yourself and being your own best friend. You will get strength and will always be able to cope on your own. You will be a better mother than you could ever have imagined because you will teach your children to stand on their own two feet.
You keep your chin up against the odds and know that you will survive. This is a great lesson for you to learn and in the future you will be a wiser person who will others will rely on. You are more than capable of rearing your children and you will be a better parent because of what has happened to you.
One last thing, allow yourself to be excited about your future. You have so much to look forward to.