"他说，这都是你脑子里的，然后我说，一切都是，但他没明白。"

Simplified Meaning:

When someone says 'it's all in your head,' they usually mean that what you're feeling or experiencing is not real and is just your imagination. The response, 'so's everything,' means that everything we experience, whether real or imagined, is processed in our minds. For example, when you feel nervous before speaking in front of a crowd, someone might say your fears are just in your head. However, this reply suggests that even real events are interpreted and felt in our minds, so they are just as important and valid. In daily life, this means acknowledging that our thoughts and feelings are powerful and real to us, even if others don't see them the same way. If someone is dealing with anxiety or stress, recognizing that these feelings exist in their mind can help them understand and manage their emotions better. For instance, by changing the way you think about a stressful situation, you can change how you feel about it, because both the stress and the calmness come from your own mind. Thus, it's important to value our inner experiences and take them seriously. By doing so, we can improve our mental health and better navigate our emotions and reactions to life's challenges.