I agree about trying not to show her you are stressed (hard, I know) Val’s (also 12) appetite has been iffy for the last few weeks, I never know when I put her food down if she will a) eagerly eat it, b) eat it after being coaxed, or c) look at it and walk away. Also, you can’t look at her while she eats . As long as she eats something each day, I try to be ok with that.
Daisy’s mouth sores may very well be what started her not eating, now you just have to get her back in the habit. multiple small meals are best, as you have found out. Also, I will say, at this point don’t worry about balanced nutrition, just give her what she will eat. Good luck!