Perhaps you wouldn't worry so much about whether you can manage the new job if you were confident that things would turn out ok if you can't? The fear often isn't so much about X, it's about the sequence of horribles that X triggers in your brain (which usually ends with me sleeping on a park bench). How about focusing on (i) the benefits you will receive even if the job crashes and burns, and (ii) all of the different things you will be able to do to get back on track if the job doesn't work out?
On (i), you'll get to try a cool new part of the country; you'll get to stretch your wings and learn some new skills; you'll get to make new friends and try a new lifestyle; you'll make a shit-ton of money that can springboard you towards FIRE (and at a minimum becomes your base for your next job); etc. In particular, realize that learning you hate something or suck at it can often be just as valuable as the opposite (my "best" college summer job was the one that clearly demonstrated to me why I was not meant to be a professor). Even crashing and burning, while damned uncomfortable at the time, can teach some really valuable lessons for the long term.
On (ii), you've got savings; your job will give you the opportunity for more savings; you will have new skills to put on your resume to help you get the next job; you will in all likelihood meet at least one person there who thinks well of you and is happy to serve as a reference/help you network; you have a bunch of other skills that you could tap for future jobs, which allows you to cast a wide net; you have low expenses because you're a Mustachian and thus don't actually need to make a shit-ton of money to manage; etc. etc. etc. IOW, you already have like 73 layers of protection between you and that park bench, and even a failed job will very likely give you even more tools to build an even more secure future, despite the short-term setbacks.
FWIW, most people significantly overestimate the amount of intelligence and skills required for a given job. I mean, if you're a quant at a big hedge fund, yeah, you need to be wicked smart and really fucking good at quant-ing. But most jobs tend to work out pretty well if you know the basics, are able/willing to learn more as needed, and generally act like a grown-up and play nice with others. So I'm actually pretty confident that you'll do just fine. But as a fellow sufferer for many years, I understand that just telling yourself everything will be fine doesn't cut it, because you just cannot possibly have an objective view of your own skills and abilities. So instead of worrying about the job -- or even about "curing" yourself of imposter syndrome -- train that big brain of yours to provide off-ramps into reality when the Death Spiral of Panic starts.
