Is "start" necessary here?
No, "start" is not really necessary in this case. "Start" is just used here to emphasize that she isn't already crumbling, but she must continue to avoid crumbling. However if you omitted "start" in this sentence, the sentence would have the same overall meaning. But you should also know that the word "crumble" is not a very common word to use in this case. "Crumble" here has the same meaning as "break apart", but it is mostly used for things which physically break apart into pieces.
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