We've all heard of a self fulfilling prophecy. You believe something to be true and it sooner or later happens. It recently happened to my friend. She mentioned to me how tired she was, and how badly she didn't want to go to work the next day. She woke up that next morning and told me she was sick.
I believe by putting negative energy out there, she created her own destiny. 
People ask me how I stay motivated, and positive. My answer to them is 1. I am not always motivated and positive & 2. When I feel unmotivated or negative, I do what I need to do to turn my attitude around ASAP.
Some of my tricks are stopping what I'm doing and going for a 10 minute walk with some calming music. Sometimes that doesn't work, so I head home to write down my thoughts. If I'm still bummed out, I take a short nap (20 mins) to turn my attitude around. If that doesn't work, knowing myself, I most likely need to just vent. I'll call a friend and just talk it out. Once I do all those things I stop feeling sorry for myself. I've done all I can do and I move on to things that are more important or topics that are bigger than myself. 
It's ok to be down and feel sorry for yourself, but the trick is to be aware and moving right along. Your mind has the power to create you or wreck you. It's in your power to choose happiness or self deprecation! Start today with a beautiful mindset.