That the relationship has become more work than fun... and not what it was cracked up to be.
I'm in agreement. This is reversed and that means the opposite of 3/Cups, which means the opposite of enjoying the party, of feeling that camaraderie. In fact, the first thought that came to my mind was "third wheel." The person who gets invited along, but feels perpetually left out. Far from over-indulging, they watch others indulging while not getting their share at all. Maybe they're even being left to clean up after the others or pay the bill.
There doesn't have to literally be another person making them feel like a third wheel, however. They could still feel that way if the person they are with is having all the fun, doing things he/she wants to do, making them feel left out.
On the the other hand, I wouldn't rule out the possibility that they might literally be the third wheel if the person they are with is hanging with his/her friends a lot, or with one particular other friend. Whether or not the sitter has said as much, there might be a "best friend" involved here who is connecting better with the boyfriend/girlfriend then they are.
I mean, really, this is a very common scenario. The girlfriend who gets dragged to whatever thing the boyfriend wants to do and he hangs with his buddies having a grand old time she is left feeling deserted and miserable. Or the boyfriend who goes out with his girl and best friend and it seems like his girl and best friend are having all the fun and locking him out of it. This is the feeling I get from 3/Cups rx. Very, very common.