Bruh, I recently got selected for an internship at a pretty high profile company next summer, and the only thing on my mind has been that they'll realise that they've made a mistake. I'm scared af that they won't give me a job offer and I'll end up with a wasted internship and no recommendations. The fact that you've gotten selected for a permanant position gives me so much hope, and obviously I'll never fully understand what you're going through, but if I was in your place, I'd take the offer as a sign that my stutter can't hold me back enough to not succeed. Do your best my guy, it's because of peeps like you that I could gather the confidence for the interview in the first place.