Can I refrain from disclosing why I left my last job?

I'm applying for a new job, and the employer has asked why I left my last job (required field). I'm not really ashamed of why I left (declining mental health and unpredictable depressive episodes) but I don't want to be seen as 'sensitive' or 'weak-willed'. Will declining to answer affect the chances of my employment, and if so, how can I word it as short and professional as possible?

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