During this kind of situation, it's hard to stay positive knowing that everything will not go to the way you planned it to be. You kept erasing all the negativity on mind just to focus on what's good and happy. But that's not what Positive thinking is all about. Positivity doesn't mean that you have to ignore all the unpleasantness situations in life, but rather it means that you must approach these kind of situation in a positive and productive way. Everyone can turn their negative mindset into positive ones. But it requires time and effort to do the job. There are some practices that may help you think and behave in a positive mannerism:
1) Harmonize yourself.
Everything starts with ones' self. Look deep down with yourself and be mindful of the things that make you feel positive towards the world and embrace it.
2) Get a Positive Circle.
Surround yourself with positive people that can support you in any way they can. You can depend on these people to give helpful advice that can get you through your negativity.
3) Get a lifestyle.
Start small and then it will become your habit. Try to practice relaxation and find out how to manage your stress/negativity. It will positively affect your mood and your body.