I had to own up to it. Waiting until "the right time" was hurting her.
I've stressed that I love her and my feelings for her haven't and won't change. I make a point of telling her this every time she brings it up.
The poly "issue" was one of the very first things that came up tonight. The counselor had never heard of polyamory. So I gave her a quick rundown on it. At first I thought this was a bad sign. But after we discussed it a bit she said "your marriage will be whatever rules you two decide on". I'm going to take that as a good sign.
But in the meantime I am trying to hang tough and be as understanding and patient as I possibly can be. My friend reminded me that I am surrounded by people who love me and want me to succeed and be happy. I try to keep that in mind when I start to despair.