Can a single thought really affect the course of life?

Yes. Thoughts lead to action, the results of action influence later thinking, and a single thought can therefore shape what follows. Here, 'heart' means thoughts and intentions, not the physical organ. Deciding before beginning that you do not know how and cannot do something may keep you from trying at all. Willingness to try again creates a chance for a different result.

In my experience of spiritual practice, intention itself also has energy. Believing in yourself and training your thoughts can help you move toward your chosen direction. Notice what you repeatedly think about. Are you willing to work to do things well, or are you continually caught in fear, resentment, and unfulfilled desire? The people and things we encounter and the information received through our senses also evoke feelings and gradually influence our thoughts, so we need to choose what we engage with.

Still, having a thought must be followed by doing something. I use work and study to explain this: wanting to work well or improve grades must motivate effort before you move in that direction. Preparing what is within your control also builds confidence. I encourage people to think, 'I will make an effort, give it a try, and do what I can,' rather than losing to their own thoughts before beginning.

Deeply rooted thinking often takes time to change. Helping someone change perspective may require explaining the reasoning repeatedly. If they do not understand, explain it differently and continue helping them understand. Caring for our thoughts therefore also requires repeated learning and practice. Hearing a principle once does not mean we will never be affected again.

From a perspective of faith, I see energy as influencing some seemingly random moments when accidents occur. But in the face of unpredictable danger, what we can do now remains staying alert and considering how to protect ourselves. Valuing intention and making practical preparations need to go together.