"Think positive" - how many times have we received this advise while confiding a serious problem to a friend or a colleague. I am guessing a number of times. And has it worked? Again if I take a guess then mostly it hasn't.
Though I am sure the advise is well intentioned but it hardly ever works. In fact something which sounds so simple is so astoundingly difficult that almost all top athletes struggle with it. Cricketers are especially prone to it. The common reasons of getting out given by the batsmen are negative thinking or thinking ahead. In short inability to control thoughts. Maybe it requires somebody of a calibre of a Sachin Tendulkar, who famously replied to a question about how he handles the expectations of over 1 billion people while batting by saying that he doesn't think about it, to actually master it.

And why doesn't it work. There are a few scientific reasons behind it. First and foremost is to analyse how does one think. How do thoughts come. And there are two very important factors behind it, one it's the play of all the brain chemicals like serotonin, dopamine etc and second is the understanding that our thoughts arise from our subconscious mind. Thus on an average we have around 60000 thousand thoughts in a day and they are mostly outside our control. Thus depending upon the way our subconscious has been programmed we'll get those particular thoughts. The one minute monkey though experiment explains it very well, where when asked not to think of a monkey for a minute, participants report thinking only about it. Simply because what we resist, persists. Thus in case we have been programmed to have low self-belief or think how we are incapable of performing a particular task or getting out of a tricky situation, these are the thoughts we'll get. And however hard we may try to listen to the well intentioned advise of thinking positively, it may be lies in the purview of super achievers like Kohli & Tendulkar and proves to be a holy grail for the rest of us mortals.
So what is the way out. Well there are a few silver linings. Hindu philosophy says that our thoughts are connected to out breath and as we make our breath slower our thoughts also become lesser. e.g. when we are angry and get shallow breaths we get a surge of thoughts but when we are relaxed and breathing is deeper, we get lesser thoughts. So regular meditation is a definite way to control our thoughts. Second method may be controlling our brain chemicals. By controlling our body movements and/or facial muscles we can bring in a change in the brain chemicals. E.g. by using happy tones and words, by forcing one to be more upbeat and energetic, by changing the way one walks : taking big steps, walking quickly with shoulders back and head held high we can become more upbeat. Thirdly there are mind techniques to reprogram the subconscious mind so that we become positive and allow only positive thoughts to come. Please find link to a short video on how mind programming can not only be used to control thoughts but also do seemingly impossible tasks.