Change your mindset?
If not you, then who? If there were someone better positioned for the role in which you feel an imposter, then you wouldn't be there. There probably are people who are smarter than you are who _could_ take the role. But they're doing something else. Why? Probably because they are actually better suited for some other challenge. And why bother with a role which isn't a challenge? It's better to vacate and allow someone who will actually learn something rather than tread the same tired ground. If we don't move, we die.
How do you know that you aren't well suited? If you feel you aren't suited for the role then your sense of value is probably out of whack. That's probably because you aren't seeing the game for how it really is. And that's fine because we all start out there.
Org charts don't tell you who the actual influencers are within the org. An org chart might tell you that Bob is the person to go to for making decisions. But people who work there know that Bill is the person you go to for really getting things done (maybe Bob is a lame duck, incompetent or just doesn't care).
Written rules create structures which have solid walls but also holes, leaks, cracks and open spaces. We tend to focus on the physical structure. But just as important is the unintended uses of that structure. The rules are important for what they say, but also important is what they _don't_ say.
Just the same, we're probably not understanding our value. We are looking at the wrong targets. By the time we do figure out the game, we'll likely move on and let some other imposter take our place.
That's good. Better to keep us on our toes. I would rather be on a plane flown by a tense inexperienced crew than a lethargic overconfident crew. While we're on the subject, there has been a history of plane crashes which could have been avoided if the inexperienced crew members were more vocal in pointing out problems to their superiors. They kept quiet because they suffered from imposter syndrome.
Just be helpful. You wouldn't be there if someone didn't think you could help. If there are smarter people working with you, then it's probably the people who hired you. Trust their decision to hire you.