some of my kids do the "I'm rubbish" thing too.
my DS1 (11) is a perfectionist, very creative and smart kid, but has proper temper tantrums when he gets frustrated. (he's just like me...
[embarrassed])
DS2 is very passionate and stubborn, and when he gets told off he starts shouting that it's because I hate him or every one hates him....ffs
before Christmas DS3 got upset because he thought he wasn't the best reader in the class - so I asked what if he isn't? no one is the best at everything, does it matter? he said not really, but he doesn't like not being the best...
I think people who are passionate about a subject or a person are very likely to get more upset if they feel things go wrong, and that's normal!
like when you cook a special meal you really want it to go well, but if a tiny detail is not right you might panic and say - well it's all ruined then..
again, it's just a perfectionist attitude - which comes from trying to please others, wanting to be recognized and feel successful/proud, and good enough is just not good enough!
you should acknowledge her feeling say "yes, I know you think it's rubbish" but ask why she thinks it's rubbish and how would she improve it (whatever "it" is) (
not sure if it matters what you think of "it", is she is disappointed she probably wants to hear that you understand her feelings, not so much what your thoughts/feelings are!
so don't say: no, you are being silly, it's lovely/good/tasty - that would invalidate her feelings and make her feel worse and even isolated.
(think of the pain of a misunderstood artist...or a teenager saying - no one understands me!)
I think she expects more from herself than is needed/required and has high standards.
she has very clear ideas about what she likes and how she thinks things should be, and I don't know if that's going to change.
But wanting to do well is not a bad thing!
she needs to understand that we all make and are allowed to make mistakes!
I always tell my kids that to expect to be able to do anything perfectly for the 1st time ever is just as dumb as expecting a baby to just get up and walk a mile, of course they can't!
every new skill needs to be learnt in steps, then practiced over and over again....what is it about 1% aspiration 99% perspiration?
and as to not being able to tell things sometimes - buy her a diary, if she can't say it at least she has a way of expressing her feelings and thoughts.
I hope this is helpful