When we work to balance our awareness on the breath, eventually it falls off. Maybe after one breath, two breaths, or even half a breath. The usual rivers of the mind (neuro-pathways or sanskaras) are powerful. Imagine the Grand Canyon and the Colorado river. All of the rain drops in the area (energy) effortlessly flow to join that river in the pathway that has been carved through time. Our rivers of focus might be anger, frustration, doubt, worry, distraction, analysis - and you might see the ways that subtle thoughts wick through to join a large familiar river that could easily sweep you away.
Meditation, and the practice of steadying the mind on the breath - or really, any change at all, at the level of body or mind - is the equivalent of taking that river and balancing it on a mountain top. The way that you participate with your mind seems so subtle, but this metaphor gives understanding of how difficult is the work. To sit in stillness and silence is something most will never do. To choose out of the familiar flow of energy and thought is something that many don't even know is possible.
The goal, however, is not to do it perfectly. The awareness will fall off the breath. And perhaps where the most strength and healing comes is in the way that you return the awareness - to the breath, or to the point of focus, or simply away from the old thought pattern. To compassionately, kindly, and endlessly return. It is this practice that changes the inner space.
Consider this: Is it wise to tell bare your soul to someone who is not trustworthy? Is it wise to tell your secrets to someone who is judgmental or too busy to really hear? No. It would not be. And the same thing is true within yourself. Our subconscious is wise. Our soul only opens up when there is safety. And if our own mind is a dangerous place - filled with judgement, business, disloyalty - we won't even open up to ourselves. The old river of anger, defeat, or false belief carries you away. And then you judge yourself for being carried away yet again.
You might have had this experience: where you are practicing well (meditation, yoga, or therapy) and there is this initial immediate burst of life and healing. But then you continue practicing well and crap starts to come up. Perhaps from inside you: the pain surfaces, or difficult memories, emotions, tiredness, etc. Or from outside you: difficult circumstances in work, relationships the need clean up, or situations that need help. The intellect says - "ITS NOT WORKING!" (the therapy, meditation, or yoga). Give me back my peace! I need a better yoga pill! (this is when most will go seek another practice to get that initial high again). But actually, the opposite is true: It is working. You have created enough safety in yourself that your soul and what whats to heal can start to surface.
You subconscious says: "Ahh, she is listening. She is strong enough to hear the pain and not be broken. She is non-reactive enough to be able to be tempered and make the right move. Let me speak these secrets, dreams, mis-alignments, and sacred contracts". Your compassion is woking. We seek meditation, yoga, therapy, for peace and then often get exactly the opposite. But this is the feeling of pain leaving. This is the work that will make your mind, heart and life a very safe place for you.
So, this is why compassionate return is important. Bringing the attention back to the breath again and again. Perhaps the art is NOT of keeping the mind still (because that is simply not the nature of life) but cultivating the quality with which you bring yourself back. Each time that you bring yourself back - with kindness, non-judgement, and deeper listening - you make of your mind a safer place and better confidant to your soul.