Gosh, you poor thing! I can imagine how you felt, being under the gun to deliver this cake, especially on behalf of your friend, and nothing working right for you.
Just so you know you're not alone, here's one of my stories. I wasn't at this wedding (had 3 that weekend), but my on-site supv told me there was a problem with the chicken and I'd probably get a call from the bride. She described the dish and it sounded just awful! We backtracked what happened, and put corrective actions in place to avoid it in the future.
The bride emailed me after her honeymoon. It was a very well written and well thought out email ... just stated the facts and the problems she observed. I called her (didnt' email ... called) and she was relieved to hear (1) I was already aware of the problem (2) had already determined the cause and fixed it internally (3) that I agreed with her that it shouldn't have happened (4) was more than willing to a partial refund.
Then .... I forgot to mail her the check!
She left me a VM to follow up. I felt horrible! Called her .... admitted my mistake and accepted her offer of her coming by the shop to get the check.
When she pickd up the check, I apologized again, and I complimented her on how she handled the situation. I recognized that she was uncomfortable making a complaint "...because none of us LIKE doing that..." and I thanked her for giving us the opportunity to make it right with her.
She left the shop after telling me she had about 3 friends getting married and she has already referred them to me.
I'm sorry for how long this got, but I wanted to share that it can work out ok. Much as we hate going thru it, it can end up ok.
It's ok to feel bad about the cake. But remember, the true test of customer service is not how you handle the initial sale .... but how you handle any problems that come up AFTER the sale.