Feel like I failed her :'(

I feel so defeated.
My 3.5 year old daughter has to go in for dental surgery for a SECOND time, first being when she was 18 months.
Her teeth are weak with little enamel but we eat healthy, treats once in awhile and always ONLY water, brush her teeth morning and night thoroughly. My 5.5 year old son has never once had a cavity, he has perfect teeth, but my daughter has already had a tooth extracted and 4 crowns, and now she has to have ANOTHER extracted and 4 more crowns!
I feel disgusted with myself, how did I not notice?! We take so good care of her teeth I didn't think more cavities were even possible! I feel so unfit to be her mother, I completely failed her.
I hate that she has to go through this again.