Rather than repressing his feelings or telling him he was being unreasonable, Tweek needed others to affirm the intense anxiety he was experiencing. Craig did exactly that, and Tweek was able to regain control of himself and overcome his anxiety on his own terms as a result of Craig's affirmation.
To be honest, I believe this episode contained an insightful social commentary on existing attitudes toward young people with mental health issues. Although Cartman definitely has his share of problems, he isn't suicidal in the least. I thought his story was a biting critique of popstars and celebrities exploiting highly topical subjects like suicide awareness to gain publicity, whereas children like Tweek, who actually suffer from mental health issues and may be at risk, are forced to hide their illnesses. Mental health problems that have been dramatized and romanticized are hip and trendy. But dealing with actual real mental health problems is stressful and exhausting, as Craig has discovered.