Diddo. My mom started by saying she thought it was gross and wrong. I wasn't seeing anyone else at the time, but I wanted her to know who I am. We're close and she's always been supportive, so I knew she wouldn't disown me or anything. The first time I told her, she also said she didn't want to hear about it and never to tell her if something happened.
Well, I ignored her request on both counts. I mentioned it again, probably about a year later. This time, she said she wasn't comfortable with it and she could never do it.
The third time, I didn't so much tell her I was poly, as I just kept using the phrase "my girlfriend." I'm not one of those women who calls my friends "girlfriends" the way my mom always used to. After telling her about my girlfriend, she finally asked "you mean girlfriend like... your lover?" I confirmed. At that point, her biggest concern was, what does your husband think about that? I told her he's fine with it, he does it too (I didn't go into details about how I'm polyamorous and he's polysexual, it didn't seem necessary). She processed that for a bit and then decided that as long as my husband was okay with it, then she saw no reason to object. Then she went on about how much she could never do that and how jealous she gets and blah blah blah.
Last time she was in town, she met my girlfriend, knowing she was my girlfriend. It went really well.
Mind you, it helps that she lives in another city and had time to process all of this without having to confront it right in front of her face.
Sorry, that was a long way to say... Don't give up, it could still work out.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker