When you classify things into positive and negative, you are stuck with both simultaneously as one cannot exist without the other. Remember that for every action there is an equal and opposite reaction - this is a law stated by Newton but Newton didn't create it. Nature did. Because Nature strives for neutrality and that impetus is driven by an underlying state of non-duality (which, alas, we can hardly experience because our thinking is limited, whether positive or negative).
Not to worry! We humans cannot even come close to this (unless we experience some serious brain damage, which I definitely don't recommend). For whatever reason, we think whatever we think (perceived as "positive" or "negative") before we can think about it, i.e. the thoughts are much faster than us and out of our control. They come and go all the time, despite us.
So in this human body we will continue to register thoughts, and it isn't about controlling the thought itself (you can't do that), it is about the weight you assign to that thought as an observer (that you could control, if you are skillful enough). A thought comes, maybe it seems positive, or perhaps it seems negative. A corresponding feeling is generated. Don't give it any importance. Don't judge it, and it will go as fast as it came (always does). And a next state will come (before you know it and of course completely out of your control). Do the same with that next thought and the feeling it generated. It made you angry? Don't judge it! Simply experience anger which, of course, will go as quickly as it came (unless you give it importance). Or perhaps the thought made you happy. Don't praise it! Simply experience happiness but don't get too attached for this too shall pass.
As you can see, it isn't about what you are thinking but about what judgements you have about what you are thinking. In other words, pay attention to the attitude with which you are observing your thoughts and the emotional states they generate. Don't struggle to think positive for the more you do that, the more negativity you bring up to balance things out. Think smart instead, accepting and experiencing life's apparent richness (happy, sad, low or high) without taking any of it too seriously. After all, life is just a game. Play smart and you win.