I'm not in a position to say whether your lawyer is right or wrong, but you may not have understood or may be misquoting. You can make a claim for undue hardship in your answer before any support amount is awarded. You should do this because otherwise you are dragging out your court case and costs. And there is no support in legislation or case law for your ex to receive spousal support for life at the age of 32 unless there is far more to this story than you say.
"My girlfriend got $1000 a month CS..." No, your children are receiving support. If you still lived with your children then you would still be supporting them anyway. Perhaps you think it would be nice if your children simply moved out and you didn't have to support them at all? You support them whether they live with you or not. You may have an issue with feeling you pay more for their support now than if they lived with you, or that your GF is benefiting from the money. Here's the reality, you won't see a formula that is guarenteed "break even" so the formula comes down on the side of the kids.
I agree that your situation is a mess, and I think that what you have said about your legal advice is questionable, and you should not be paying SS any longer. However at least some of your situation is the result of choices you made. Why did you choose to support the first woman in the first place? Why did you choose to have more children if you couldn't afford the support you were already paying? You stepped into a certain type of lifestyle, you accepted dependants, your first ex doesn't sound like an honest and reasonable person. I do hope you are able to sort this out in a fair and reasonable way, but the courts didn't force you to get married in the first place.