The mind is a place where channel of thoughts run through. It is not every thought that you allow to take root. For you to have a stable mind you have to learn to manage what goes through it because a stable mind aids productivity.
Never absorb a thought that will affect the stability of your mind. The mind is powerful as much as it can create it can also destroy but it all depends on the access of thoughts. Watch the thoughts you process in you mind and be conscious enough to stop the unhealthy thoughts. When you are happy this second and the next you are sad it simply means within few seconds your mind has not only processed but absorbed thoughts that have passed a wrong signal to your entire system.
Sometimes before the blink of an eye you'll realise how your thoughts have gone out of control. That is why you need to let God in so He can help you handle things beyond your control. He created you so He can handle your mind. Stop letting the wrong thoughts be what resides in your mind. If you desire to be healthy and productive, it starts with your mind. It is what you have you give out. Get the right feed into your mind and produce the right result. Don't weigh your mind with the things God has taken care of in the cross!