I know what you mean, I'm feeling it right now. I used to have these, maybe lasted a few days even a week, then the next week I was fine. I think it is part of growing up, although that negative feeling can be changed into positive in almost any situation. Anthony Robbins taught me how to deal with it, it went a little something like this:
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Every emotion is not based on a situation, but your interpretation of that situation. Steps to overcoming emotions:
1. Identify the emotion.
2. Appreciate it and respect that emotion.
3. Become curious
How do i really want to feel?
What would i have to beleive in order to feel that way?
What am i willing to do to be able to achieve that feeling?
What can i learn from this?
4. Become confident that you can handle these emotions.
5. Come up with different situations to overcome these emotions. Rehearse!
6. Get excited and take action, go and DO something about it. Reinforce the things you rehearse.
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This is basically what I did every time I felt a negative emotion. It really does work if you take time to learn how to do it. It's basically reframing as taught by David D. (who probably got the idea from Tony Robbins). I have spent almost 6 months with about 80% less negative emotion than I had before, simply because I didn't know how to handle it and transform it into positive.
Note: Or maybe you're just a perfect candidate for Scientology
P.S. - You can get into negative states just from your internal dialogue (ID). Try not to say anything negative to yourself like "Why am I feeling like such a loser?". No thinking is better than negative thinking. Alternatively you may ask "How make myself feel more positive?".
These types of things may seem insignificant but once you can turn your ID into highly positive ID then you're well on your way to overcoming slumps in your mood.