I do this too sometimes. I deliver the order and every insecurity I have about it feels like it's just flashing across my forehead in neon lights. And, before the client gets to read my forehead, I feel like I just gotta point something out that I had trouble with or make an excuse about some aspect of the product. From my end, it's me being nit-picky and a perfectionist. From their end it may look like I am not capable and therefore lose confidence in my business.
I've learned to keep quiet. It's almost like we have a body image problem. You know those girls who look in the mirror and see fat. When really, they are just a skinny rod? It's sorta like that.
Anyway, I think the cake is lovely. I like the bright colors & it's clean. The only difference between that cake and yours is "more". You just needed more. BUTTTTT, she said she didn't want more because of the wording. And, your writing is great. I wouldn't fret.
If you feel real horrible about it, make her a little cake or cookie as a "thanks for being a client". No excuse attached. Just a nice gesture.