Some of this has already been really helpful. I am still getting to know who is who on the thread, so apologies for not responding to everyone else's posts - I don't want to say the wrong thing at this early stage.
It's mostly reassuring to see that I am not the only person in the world in my position - it feels as if I am sometimes. I keep reading about how many marriages end in divorce, and wonder why I'm the only person I know (and why my children are the only children in their classes) in this position. It doesn't help anyone, but it does make me realise I'm not alone, and that the way I feel is normal.
I have really good days (today) and really bad days (yesterday). On the bad days, I can't understand why, four months in, I still feel so shocked by what has happened, and can't imagine ever being happy again. On the good days, I think that life isn't actually that bad. I spent a lovely day with dd today (my mum looked after ds) - we went out, had sushi for lunch, bought an aquarium for her birthday present (6 next week!) and went to a museum. We had so much fun and on the way home she said "Mummy, I didn't think I'd ever be happy again when Daddy went but today I do feel happy" which made me cry. I had an email from my Dad offering to pay off a chunk of the mortgage once the house is transferred to my name, which is an incredibly generous and really unexpected offer, and will make a huge difference (I know how lucky I am...).
The problem I have is I wasn't unhappy in my marriage, and I didn't know there were problems. So I am definitely grieving for the relationship I thought I had, while trying to get my head around the fact that it wasn't what I thought it was, and hadn't been for a long time. So there's no sense of liberation or relief, the way some of you describe - he clearly has turned out to be a very weak and immoral man, but when we were together he treated me well. Or at least, I thought so - I have had a lot of people tell me they don't think that was the case, so maybe I need to re-examine my expectations of what it looks like to be treated well.
Tomorrow, I've got lots of fun stuff planned while he has the children. I'm determined not to waste the child-free time I have - I need to see it as a positive thing, rather than sitting around pining for them. Would be really interested to know what other people do with their spare time.