Chanting and mental disturbance
When chanting Hare Krishna we practice neglecting the mind and paying attention to the sound of the holy names. It doesn't matter what crazy ideas the mind comes up with. Our concern is just to hear the sound vibration of every syllable of the Hare Krishna mantra. When we notice that we're paying attention to the mind and not the mantra, then we bring the mind back to the mantra.
Because we have paid close attention and obediently served our mind for as many as millions of births, the difficulty of ignoring it and focusing on the sound of the mantra should not be surprising. But just as a mother patiently keeps her children from causing trouble—although they do the same stupid things again and again—we must patiently control our minds.
It's important to always try to control the mind by hearing the sound of the mantra attentively. In the course of time, we will improve at it. Krishna will appreciate our efforts and give mercy in the course of time.
As long as we're chanting, we should be hearing. Try to focus on one mantra at a time, and then part of the mantra at a time, and then a smaller part. Then you won't have time to meditate on the distracting thoughts of the mind and you will become purified by hearing the mantra.
First, make a determination to do nothing else except focus on the mantra for this period of time. Then focus on the sound of one mantra at a time, and go on to the next. Don't lament about the past or dream about the future, just listen to the sound of the mantra, whether you like it or not, because it is Krishna's desire we chant His name in this age. Set a fixed number of rounds to chant and then gradually increase.