Hey OP, I've been there, got the t-shirt and am now (at work at least) all the better for it. I changed jobs like this about 4 years ago now. For the first 3 months had serious impostor syndrome but now a few years on its all good and I have to say it's the best career move I ever made.
Remember, if a company makes an investment in you, not just with the job offer but also the time it takes to interview you and process your application they see something in your thats worthwhile and that should help to give you some confidence!
Good luck and enjoy your new job!