Marcus Aurelius, Meditations, 5.16: Your mind will take the shape of what you frequently hold in thought, for the human spirit is colored by such impressions."
You can view your brain/mind as any other part of your body, if you sit all day, the curvature of your spine changes, if you work with your hands all day, you will form a callus. If you have negative thoughts all day, your mind will interpret encounters as negative. Most people view this is a half glass full or half glass empty analogy, but it also can be extrapolated to even at least you have a glass at all. Although negative thoughts are bad, stoic philosophy consistently uses 'negative imagery' to bridge your way to more positive thoughts in the future.
Its easy to say to be positive, but we are truly tested when we encounter things in our daily lives which greatly trouble us. For example, if we get in a car accident, most people become anxious/nervous/shaking due to event and all the issues that may follow. But replacing in your mind with the thought of a worse outcome (such as severe injury, death), can help you have a positive response to a very unfortunate event.
Today as you go on your day, when you come across something that is obviously bad for you, try to think of a more negative event that could have occurred in that same situation, and start the process of removing negative thoughts all together and focus on the positive.