Addressing Your Problem
I usually feel better when I am pitying myself and addressing my pain. It will often help me get into a better mental state. However, it does not help unless it is a pity with slight compassion as against pity with aversion. In pity with aversion, we are battling against how our life is. This is a similar consequence of ramming our heads into a wall - useless and harmful.
You must not be pitying yourself angrily. If you have thoughts like, "I hate the fact that I cannot find out how to feel better," or, "The pain better not stay around," you aren't going to feel any better, but worse.
Changing Your Environment
This is the best coping mechanism that I have found, but different people have different techniques. Sometimes I have to make myself make the first move. Do something, absolutely anything (that isn't harmful, of course) will make a bad day much brighter. My way of modifying my surroundings is by going on a drive. You should take a shower or go for a walk.
A way to change your mental environment is to get laughing. "Laughter is the best medicine." It may be slightly cliche, but there are truths in the sayings. Usually, just go through the day. There is normally one point in the day that is truly uplifting. Surrounding yourself with friends is what gets me laughing.
These Feelings Are Not Permanent
Always remember that tomorrow is another day, another day to improve yourself. Put on a funny film or video, try and learn something new. Try and keep your mind off the thing that caused the bad day in the first place. They won't instantly brighten your day, but it'll certainly be a start.
Emotions aren't permanent. Moods aren't permanent. Bad days aren't permanent. It will all pass, just give it time and help your body speed up this process by putting yourself in a calmer mood. Try meditating. I know many people see meditation and think, "How would this help? I would get distracted too easily," but trust me - it helps.
