Struggling academically and emotionally in my IO PhD program

Sorry if this belongs in r/academicpsychology or some sort of academic therapy subreddit, but this was the first subreddit I thought to post in. I'll try to get this out without being too touchy-feely.

I'm in my first semester as an IO PhD student. I've felt a profound sense of inadequacy ever since I started in August. This is the only program I was accepted to, and I was only considered because the 6 or 7 applicants in front of me declined the offer. The other students in my cohort were the first ones to be considered, however. Without ever meaning to, they've intimidated me since the first day I met them. But I knew it was unwise to constantly compare myself to them, so the intimidation factor eventually diminished.

Since this was the only program that offered me a spot, I had to accept faculty with specific research interests that didn't exactly complement my own. That's not to say mine are of a different specificity; I don't know what I want to research, for the most part. We also don't have to pick an advisor right away, so I'm stuck in limbo.

This past week I got back my first exam and assignment grade in one of my classes. The raw scores are posted for us all to see, so I could see that I got the lowest grade out of anyone in my class. And ever since then, I've been ruminating to an unhealthy extent. I'm having trouble sleeping. My ability to concentrate has begun to diminish and it's worrying me. I'll spend twice as much time reading an article because now half of it is spent brooding and imagining self-deprecating hypothetical situations.

I know about the impostor phenomenon and I was probably going through that at one point, but this seems like a larger problem. Has anyone gone through something similar to what I'm experiencing or knew anyone who went through this? Does anybody have any advice?