Wow, I can see why he's divorced.
It's time to stop asking him to help. It's time to TELL him how it will be. Tell him that you will no longer be picking up the kids from school on a Friday. Tell him that you will not be taking his daughter to sport on Saturday. Tell him that you're doing your own thing on Sunday and cannot mind his kids while he plays golf. Yes, he will have a hissy fit and blame you, but stand your ground.
To be honest, you need to do this as much for his kids as for yourself. Absolutely no offence intended because you're doing a great job - but I bet his kids want to spend time with their dad...not hang out with the step mum.
I suspect that you standing up to him could send your relationship into full crisis mode, so be prepared for that. But let's be honest, you can't continue being used like this.
Who's house are you living in?
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.