This is a quote from the Dhammapada, which a the collections of sayings of the Buddha written in verse form.
"Your worse enemy cannot harm you,
as much as your thoughts unguarded.
But once mastered, no one can help as much".
This is so simple and so true.
But to master our thoughts we must first catch them. Out thoughts are so much a part of us that we are often unaware of actual content Sometimes they are not even in word form. The first we may know of them is that they have created a feeling and a chain reaction of other thoughts. We need to learn to look out for them. Learn warning signs of their presence.
When caught we need to be gentle with the thoughts. Befriend them. Understand why they are there, when they started and why they have such power.Be respectful... they were once useful and possibly true. Be respectful because they are part of you.
Only then... master and train. Initially wild thoughts respond best to simple commands. They need to stop as soon as possible before they gallop off in their habitual way. These commands are ones you decide to be your mastery tools. They could be words, immages or actions.
Some examples of mastery words..."Proof," "WAS true", "This IS life", "New ME"...you invent one.
Examples of mastery images...a star, top of a mountain, your hero...again you invent.
A mastery action may help...walk away to another place, stand taller, breathe deeply.
Now replace the old with the new...now I think this...Be as specific as you can in thinking the new thought. Eventually just the reminder will correct and you will have mastery.
But...don't lock that this is it for ever. Life changes and your thoughts need to adapt. Stay flexible. True mastery is flexibility.
We can all improve our mind control.
So I encourage you...with the words of the Dhammapada..."Your worst enemy...."