Mind-the-Mind effect

It becomes frustrating to not have control over your words in spite you know in advance that which words will do good and which will do badly. I asked the solution from one of the Saint and he told me that " it is a mind-the-mind effect where we forget what to remember but surely remember what is supposed to forget. Always tell the mind what is to be remembered and not what is to be forgotten". 

I thought about the same for many days and then realize that "Knowledge alone will not make the difference but the practice of using knowledge will make you wise". So I am going to practice the same in the coming days and would be great to know your experience as well about it.