I reckon that after so long together you have to at least give it a go.
Remember what you saw in each other in the first place and recall all the good times you have had together.
I don't know how old you are but for me, 18 years would be half my life - and you have two children to consider. I'm not saying you should give it a go just because of the kids but you loved each other enough to spend 18 years together, surely that love can't just disappear, its probably buried beneath loads of other mundane stuff!
Try and work through the tough times and see what happens. It might not work out but at least you would be able to say that you tried.