Op, there was a long thread about this a little while ago so worth searching.
I think step parenting is uber parenting, it makes 'normal' parenting seem easy.Add to this a toxic ex and a dad who won't step up then you have an unbearable situation.
You have 2 young dcs, so 4 children at times and a husband who goes away a lot? .That is stressful in itself.
Dh's ex is toxic, much more when she was unhappily married to her 2nd husband Unhappiness with her life seems to be the core reason for the toxicity.
Court defined contact helped to lower the conflict but she ramped up the vicious personal behaviour.I was her target (not ow as she was engaged when I met DH).
She saw no downside to making her DD bully me, DD later confessed that the ex and dd spoke about what she could do & say to be hurtful.Dd was 8 at the time.How a mum would coach a child to do this was beyond shocking.
DSD has half siblings with her mum but when dh & I had a child DSD was highly jealous.This is something you might need to recognise as I hasn't appreciated how much jealousy there was.
Not sure we fixed it but talking to dsd helped (she denied it at first) but her behaviour showed otherwise.Dhs ex would encourage the feelings of jealousy however from my perspective dsd was/is highly indulged, much more so than my dc.
Dsd is now older and it's still not ideal but I have learnt to deal with it better.I take my dcs away for breaks if feeling overwhelmed.I could be bitter than I have to do this but choose not.I disengage from the Disney parenting (biting my tongue often but not always!).dsd is not better for the indulgence she has had.My dcs who have given given less and corrected more frequently are nicer people.That will be an advantage for them in later life.
I think you need to find ways to cope, accept you can't change the ex, don't tolerate rude behaviour, be firm but fair over DSC comments 'thats not kind behaviour' is often said despite me wanting to rant!
I don't run around after DSC unless I want too, don't feel you SHOULD do more for them as it leads to resentment.Make your dh step up.DH is less Disney if he has to do all the running around for the DSC.
It is your house so you have the right to put boundaries in place.Try changing your behaviour (detach, take breaks, practice stock phrases) rather than telling your dh to change (because when he won't it just draining and upsetting).
Give it a timeframe and see if it's still as awful.Then you know you have done all you can.