"Expectations of" is probably the most common, but all three can be correct.
I have high expectations for the business.
I have high expectations of our success.
There may be an impact of my expectations on the quarterly results.
I don't really like these sentences. My own personal style is to write them differently.
I expect much from the business.
I expect we will succeed.
What I expect may affect the quarterly results.
In your example, I would probably try to use expectation of, although there's much I'd have to change to make it work. For example, "justify" does not mean what you seem to be trying to make it mean. It usually means to try to defend some bad action by saying it was done for a good reason. Example:
The thief tried to justify his theft of the bread by claiming that his family was starving, but the court was deaf to his pleas.
Instead I would say something like
... she did not live up to my expectations
... she did not meet my expectations
You can use either the plural expectations or the singular expectation, although the plural might be more common. Here I prefer the singular, since you state specifically the one thing you expected.
... however, she failed to live up to my expectation that she would become more fluent.
If you want to be clear that it was your expectation that is at fault, and not her own failure, then perhaps:
... however, my expectation that she would become more fluent was not met.
... however I think I had too high an expectation that she would become more fluent.
Again, I would write this using the verb:
... however it seems I expected too much from her as she did not become more fluent.