Polly! Once again, brilliant.
I would add that as someone who has also been in this situation, there's something particularly satisfying about secretly loathing someone in your friend group, no? Scratches some evil little itch, and it's easy to let it fester; very tempting stuff.
The best solution I've found is, as Polly says – just be cheerful, busy, and unavailable. If they passive-aggressively confront you, Play Very Wide-Eyed and Dumb: "I swear it's not that, I'm just so swamped!". Continue to be unavailable. Eventually, even the craziest Jessicas will find a new target.
I was once the 'third roommate' in a secret roommate romance. I only found out they were sleeping together after eight months in, when he came home with some other girl and she flipped out. The police ended up being called. Good times!
LW, my anxiety comes from a different source, but I could have written some variant of the same letter. Just today I was saying to a friend that I could be sitting in a room full of fluffy kittens with a bajillion dollars in checking and Clive Owen rubbing my back and I would still be bubbling with anxiety at some level.
This is great advice from Polly (as ever). And may I just extend my very real love and hugs to you across the wide interwebs. There is hope out there. And you will be surprised at how much you can overcome.
Best Ask Polly ever? Best Ask Polly ever, ever, ever.
@Alex Balk But.... but all of the rest of this is just garnish!
WHERE IS THE BLUE CHEESE, YOU MANIACS?