a sound mind is a mind that is at peace; free of worry, fear, anxiety. A mind capable of good and rational thinking thinking. A wise mind.
Jesus gave us peace. He gave us His peace. The seed of peace was planted in our spirit when we got born again. We are to let the fruit of this seed rule us.

Experience can increase one’s empathy level, help balance emotions and cognition, help manage expectations, provide a wider view on things, can help with planning, … and more.

Unlike Cain, a Christian has a new spirit (2 Corinthians 5:17) and can therefore determine what happens in his mind. He can rule over his mind by consistently feeding his spirit with the Word, that way his spirit has dominance over his mind. He can determine his thoughts (Philippians 4:8), and he can control how he reacts to his feelings (1 Corinthians 9:27).