The mind is "many-pointed," like the spikes of a porcupine's coat. Your job is to focus the mind, to make it one-pointed, and to eliminate obtrusive thoughts. When you repeat the mantra watch your reactions. There will be a never-ending flow of thoughts and feelings. Watch them, and keep repeating the mantra. Purchase the book Happy For No Good Reason at The Ashram Book Shop.
You can take Swami Shankarananda's online Learn to Meditate course which accompanies these meditation audio tracks here.