I am worried what it does to her self- esteem
The fact her "brain doesn't compute properly" may mean many things.
For all we know, she may have some underlying medical condition that should be investigated just to be sure she is alright.
Girls sometimes have mental confusion and slower response due to simply anemia, for starters.
Has she ever been tested for rarer forms of anemia, for example?
this is for infants and developing fetuses, but it relates to children and adults, too. I chose this link because it offers also a lot of other diet aspect
(that's just as interesting information bit, not to imply anything) :
Iron deficiency eventually leads to anemia, with insufficient oxygen reaching the brain. The anemia can cause fatigue and impair mental functioning.
Read more: Nutrition and mental health - children, causes, functioning, effects, therapy, person, people, used
Nutrition and mental health - children, causes, functioning, effects, therapy, person, people, used
This is just an example how sometimes the body is not working properly and you are not aware of it not that your stepdaughter has it.
btw Iron deficiency may happen even with the proper diet - if the "gut" is not absorbing food the right way.
I know because I have it. So you never know.
Or she can be more of "brain left side" thinker than more common "right side",
and those people have a harder time using the 'cookie cutter' teaching approach they have in schools, for their brain process differently.
last but not least, if indeed there is nothing our of the ordinary
and she just does have harder time getting the hang of time,
I am sure there are areas at where she excels,
and this should be explored and given special attention and praise.
Fuzzy