What is the main point of the physical posture?
To maintain the awareness of the 7 points.
What should you do when your mind is distracted from the anchor of your natural breathing?
Whenever your mind is distracted, then simply come back to the object again and again, without any reaction towards your thoughts.
How can you overcome the difficulties and challenges you meet during the practice? What is the most important tool to develop in meditation?
We must accept challenges and difficulties on the physical and mental level: this is called: patience. Patience is the most important tool to develop in meditation.