A key part of Mindfulness is acceptance. We accept that we will have thoughts and feelings that are negative in nature. Our goal is to allow them to pass by, observe those thoughts but not to allow them to affect us - we are detached from this kind of negativity. Sometimes events happen that affect us to the very core, and its difficult as human beings to not be affected by them.
When this happens, we need to find a way to let the negative emotions out when they arise. Don't keep them bottled up or they will explode at a bad time.
Hit the gym, go for a run, a long walk, a bike ride, punch a punching bag, or even a pillow on your couch. Hammer something, take something apart that needs to be dismantled. Channel that energy and give it an outlet.
You could also find a way to express your emotions in art. Write a song, take a series of photos, create something that allows what is going on inside you to come out.
Other options are to just get in your car, drive somewhere secluded, and scream your lungs out(in the car, that is.) don't abuse the trees with your language.
Other things I've done to pass time when I'm really upset - soak in a bathtub, with headphones on, listening to something with a hard beat, something that represents what you are feeling. If you can concentrate enough, read a book that you can immerse in. Also, write a lot, just put your thoughts down. I never sent anything I wrote, and it along since disappeared, but it did help to write....anything.
The key thing is, don't do nothing. Your feelings are there, don't ignore them. If you can't accept them, Instead use them, let them out constructively, or release them in a safe way.
And slowly the negative emotions that focus on you will get out, and the positive emotions that focus on others will be free to surface and take over. That's when you will get your second wind, and most of the battle will he long behind you.