I agree, and I don't usually like whiny characters, but Subaru's an exception because it's part of what makes her so fascinating. She's a prima donna, and not only in the sense that she's the principle dancer. She has the personality of a prima donna, which the Merriam-Webster dictionary defines as "a very temperamental person with an inflated view of their own talent or importance". Apparently, the mangaka wanted Subaru to embody this personality. Also, her inflated ego is somewhat inherent, but stems from her vulnerabilities. When Subaru's traumatic experiences resurface (e.g her brother's death), the egotistical personality acts as a survival mechanism, which motivates her to dance. Therefore, Subaru wouldn't have become a phenomenal dancer without it.
It's frustration. Since she's a "natural talent", she wants to quickly better herself, she expects more. She has a way of thinking which is different.
Geeeez shes such a whiny little brat. And she cries like allll the time when she doesnt get her way.