OP I know how you feel, and at first when I heard what I'm about to say, i thought "what the hell, how can that be true". i couldn't get my head around it and understand its true meaning until I tried it.
"You choose how you feel".
If your car breaks down, do you choose to get angry and feel despair, or do you think 'you know what? its beyond my control, its happened, I might be late to my appointment/work, but theres nothing I can do, so I will choose happiness instead.'
If an exam doesn't go well do you feel upset and down with yourself , or accept that it didn't go well, and theres no nothing you can do to change it at that second, so choose to accept it, and move on, and do something to change it into a positive?
Its all about perspective. If you feel negative about something, you're not going to get anywhere, whereas if you take positive action, and have positive thoughts, you are more likely to have the positive outcome that you want.