The sentences with incorrect parallel structure in your passage are:
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"People commonly feel sad or feeling withdrawn because of failure."
- This sentence should maintain parallel structure by using the same form for both actions: "People commonly feel sad or feel withdrawn because of failure."
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"If you stop trying, you will stop learnin."
- This sentence should also use consistent verb forms for parallel structure: "If you stop trying, you will stop learning."
In both cases, the verbs should be in the same form to create a parallel structure.