Unfortunately, there is nothing you can do about this one, especially as it's still very early days of you an your DP being a couple.
Yes, his ex is being a pain. Yes she's unreasonable and difficult, but you have to leave it to your DP to sort, or not, as he sees fit. If she disrespects him in front of the kids, its up to him to stand up for himself, not you. If you do, you'll make him appear weak and that will make him angry with YOU. All you can do is support him and not add fuel to whatever fire she's choosing to light. Don't bad mouth her to him or the kids (tempting though it will be), be polite to her and just go with the flow. With the passage of time, it will get better (in my case it took roughly 8 years!). So she plays the victim, big deal. That's her decision. The only person she's making look foolish is herself.
As to time for yourselves, well, that will come in time too. His kids are teenagers so it won't be long before they do their own thing more and more and then fly the next - as yours have done.
So I guess in answer to your question, yes, you do just have to grit your teeth, smile and carry on. Just don't let it drive you crazy, it's a total waste of time and energy. 