For quick change in your thoughts you can do the following things.
1.Take a long breathe till you get fresh again.
2. Shift your focus from that negative stuff. Start thinking of another topic which is positive or gives happiness to you.
3. Start saying positive affirmations in mind.(You can say loudly if you are alone or you are driving a car or bike)
4.If you are sad or angry and then you are having negative thoughts,drink a 2 glass of water so your body temperature and BP will get back to normal.
5.Start listening relaxing music
6.Get detached from the negative friends and family members for some time at least !
These will definitely help you to turn your thoughts positive ! For more details you can watch my video on a positive thinking. Visit this link. https://www.quora.com/I-have-totally-no-motivation-in-life-anymore-I-feel-tired-everyday-and-dont-want-to-do-anything-But-this-is-not-what-I-want-how-can-I-change