Although our minds have positive and negative thoughts, these do not affect the nature of the mind. It is like the good or bad of the weather does not affect the truth of the sky. The thought of birth and death is only a thought, not the mind itself, but a phenomenon of the mind. Rain and sunny weather are just a phenomenon created by the four major causes, not the sky itself. All joys, sorrows, and sorrows are just illusions, you shouldn't beleive in them at face value. Knowing your own self is the only way to let your thoughts dissolve.