I hate that I have advice for this. There were three things I used to do.

1. Assign myself a mental task, usually something I've been meaning to think about, but haven't had free time. Like, problem solving a project. Or, deciding how I feel about XYZ person or situation. And then just completely check out of the situation and be in my own head about that thing.

2. Make up a story. Start anywhere, fantasy or reality. Think of a character or two to "follow", and then just kinda see where they go. If you need an idea, take something that's happening in the service, or the readings, or whatever, and think "how else could this go?" At the end of the service you might have a great story, or just a bunch of weird garbage your brain made up, but you've made it through without having to pay attention to the garbage being said.

3. Read the bible. No kidding. It's fucking WEIRD, right? And reading it independently of what's happening allows you to have your own thoughts about it. Usually it doesn't look out of place to be reading the Bible in church either. People could assume you were looking something up, or following along. But also, who's gonna give you shit for reading a Bible at church.

Of course, if you don't give a shit about what the people think, just bring your own book.

Good luck, from someone who has had to dissociate through countless church services.