I have only skimmed this, but I dealt with x hubby by thinking about it as a triangle. This really helped me to be civil (most of the time) and dissociate myself from his general naffness and then I moved on... ok this was 10+ years ago, but it worked at the time too...
The triangle works like this - I am at one point, he is at one point, the kids are at one point.
Xhub and I have (?*!) a relationship - one side of triangle, and I have relationship with kids (another side) and he has a SEPARATE FROM ME relationship with kids...
When I realised this HE had to take responsibility for being rubbish when he let the kids down and I stopped feeling SO responsible for his poor behaviour - it stopped being my fault and so I stopped feeling guilty and could just feel mad (on the kids behalf). Also I could still be annoyed about the merry dance he led me, but it took the kids out of that part of the equation.
I love my kids to bits and if it wasn't for him (ironic I know) I wouldn't have them. I have tried hard not to rubbish him in front of them EVER, even if I say stuff when they're not around, as he is a part of them, no matter how annoying that is, and if I run him down in front of them I feel I am being unkind to 50% of each child and will ultimately make them feel bad.
givemehope - if he was awful and abusive should your ds have the right to supervised visits??
And lostdad - I don't know full situ, but you could sort a visiting order thru courts...hard I know, but visitation orders can be made to work and if ds comes asking questions in the future he'll know you wanted to see him and made every effort possible.