First let me say that we are always our own worst critics. Our eyes look at cake in a way that is different than everyone else. You are the only one who can say "it's not my best".
That being said, if you are truly unhappy with the cake, then the right thing is for you to make contct with the bride before she calls you ( if she intends to). Only the two of you know what was envisioned. If the final product doesn't match it, an acknowledgement is step one in making it right.
I truly doubt that it 'ruined' her wedding, but since there is no do-over, it might have been disappointing to her. A refund doesn't take it away, but paired with an apology, it is a huge reflection on your integrity.
The best thing to do is to tell her that you don't feel it was your best work, you would like to offer her a refund and ask her if there is anything you can do to make up for it. She may say it was fine, no refund necessary.
We're going to have to suck it up at least once in our careers. Someone said my cakes look great but don't taste good. I know that's not true and everyone has their own personal taste, but believe me, it cut to the bone.
Hey, if we were all perfect, pencils wouldn't have erasers. Don't be discouraged!
Hope that helps!