oh I sympathise with you I really really do.
In the same boat. So hard. Try counselling, as others have said.
It's very hard when you see them regularly because of the children, and to see that they have no feelings for you. My goodness that hurts.
Whatever you do, don't sleep with them (See my other threads!) it will only take you back about 50 steps.
The pain of breakup is so hard, especially for the ones left behind. You're ex is in a different place to you and that's really hard to accept. I know it and feel it. Its awful. But, think of it like an illness (the breakup, that is, not the relationship!) and that you will have good days, bad days and one day a complete recovery! That's what I'm hoping anyway.
Find a hobby or just escape when x comes round. Get out of the house. Don't hang around trying to talk about things or hope that they'll look at you and go "good god, she is the best thing that ever happened to me, I want her back" because it won't happen and even if it did, you don't want it to happen on those terms. Believe me I have been there and all it's done is crush my soul and ruin my confidence.
Get out, make friends, do something, exercise.....It won't go away immediately because you're grieving and you still have feelings for your x. But be kind to yourself.
I know how awful it is. I am still going through it myself and it's rubbish