Growth isn't inherent to the struggle, plenty of people are bitter after facing adversity. The only way to grow is to reflect on the struggle and try to make conscious growth after the fact. Also, everyone is going to define their struggle differently based on their beliefs, values, and experiences, and I don't think Nietzsche succeeds at trying to put such a subjective concept into objective terms.
Weak people are bitter after facing adversity. Growth is not inherent to struggle, but struggle often gives opportunities for growth and change, and struggle comes with any difficult to do, learn, and master. Nietzsche would not begin to do this really. His seeing subjectivity as something everyone is forced to deal with would goes against him defining struggle objectively for all people. Struggle can only be defined subjectively.I don't think Nietzsche succeeds at trying to put such a subjective concept into objective terms.