When you focus on the breath, using the breath as a leash to tie the mind in place so that it doesn't go wandering off, you have to use your endurance. That is, you have to endure pain. For example, when you sit for a long time there's going to be pain, because you've never sat for so long before. So first make sure that you keep the mind normal and neutral. When pain arises, don't focus on the pain. Let go of it as much as you can. Let go of it and focus on your mind....For those of you who've never done this before, it may take a while. Whenever any pain or anything arises, if the mind is affected by craving or defilement, it'll struggle because it doesn't want the pain. All it wants is pleasure.