She is probably very rattled by the whole thing. Most people are terrible at understanding their own feelings, especially older folk who would have been more likely to be taught to shut up about their feelings from childhood. If you don't understand your own feelings, you end up doing all sorts of bizarre things when tension is running high. I would assume this is what your mum did.
I would just go with "oh my, mum, so sorry you're upset, I hope you know how much I love you, yes it was a fuckup this year, it's hard isn't it? next time will be better, always remember how special you are to me," etc. etc. until she calms down and accepts it. Then move on and never mention it again.
It sounds as though she isn't at her best, but at the same time, you don't need to spiral into a massive existential crisis over it. You're a grown up, even if she is angry with you, that doesn't mean you're a bad person, it just means she's having a shit day and, unfortunately, being a toddler about it.