Yes! I am in a similar situation, accepted a spot at my first choice program this year and now I have no idea what I'm going to do. I knew it was Imposter Syndrome but I also recently had it described to be as an anxiety hangover: I've been so gosh darn stressed the past few months (a year even) that now that I've gotten in that anxiety has no where to go and I feel almost empty. So I've been subconsciously looking for things to stress out over (like getting on grad cafe, for instance lol). Don't get me wrong, I'm still thrilled to have gotten in! But now that the excitement is starting to wane, I've begun to feel that emptiness creep in since this is something I've been anxious about for a long time and that anxiety is now gone. Pretty sure this is a fairly normal thing, and I would also recommend going to talk to someone if that's something you can feasibly seek out.