What I was referring to was that I thought I was ready and wasn't and in the process ended up hurting my partner and the woman he was interested in. I was thibking about how to fail like this without hurting others too much.
But after I wrote the original post I learned that they already had sex, even though he had promised to me not to do anything without letting me know what direction things were headed. He did this at a time when I didn't even know he was interested in her, so that's a major dealbraker. I have felt bad for two weeks thinking I'm just not good enough to have a polyamorous relationship. And all this time it was him.