I have recently been in the same situation as you, and you have my heartfelt sympathies.
I am sure, like me, you always thought that when you got married and had the children you would be together for life - I certainly did. It took me a while to get used to the idea that I was now going to be a single mother to 3 children and I found it hard to accept the fact that my children now came from a "broken home".
When me and husband first separated, he moved into the loft (spare bedroom) and my DD1 (9) asked questions - unfairly, we told her that it was because the new baby was waking daddy up and daddy needed to sleep as he has work in the morning (I was on maternity leave).
Husband moved out beginning of September, after leading separate lives (but still living in the same house) since February when he told me for the second time we were "over" (or New Years Eve - when he told me for the first time we were "over".
My biggest concern through all of this is making sure my dear wonderful children (10, 5, new baby (then!) (now just turned 1) grow up to feel loved, secure and stable. Although mummy and daddy are no longer together, we still both love them all to bits - we just can't be together any more.
They see their dad Wednesday night and overnight on either Friday or Saturday until mid-afternoon the next day. For the last 4 weeks, they have been sleeping at his girlfriend's house and they are absolutely fine about it.
DD1 is obviously aware of what is going on (she is 10 next week) but I am trying my hardest to be as truthful as I can with her. Last night she said she felt sorry for her baby sister as she would not know what it is like to grow up in a family where the parents are not divorced. (We are not divorced yet, but are definitely separated, although not legally).
My DS1 (5) is just so accepting of the situation. I think it helps that, these days, it is not that rare for children to be brought up by one (birth) parent and it certainly helps my son that his best(est) friend only lives with his mum.
As long as my children see their dad regularly and they know that we both love them, then they will be fine - as I am sure yours will.
It is a horrible thing to be going through (sometimes I feel I have got through it, other times I most certainly feel I am still struggling). The hardest times are anniversaries, birthdays, etc. and especially now Christmas is looming. For me, it won't be much different to past years - staying round mum's Christmas Eve, spending day there but the big difference will be that my H will not be there. When I imagine Christmas, in a fantasy world, I imagine nice open log fire, family sitting round tree, christmas tree lights on, nice family atmosphere basically. This year, we will have the real christmas tree, decorations, etc. but it won't be the happy family celebration I want for my children but we will still have a nice time.