Recently, I dreamt about a mistake I made in a college football game played over 20 years ago. Interesting... I made a lot more good plays than bad but I still wind up dreaming about the bad two decades later. Which really isn't that surprising. Any coach or ball player will tell you that the games they lost stay with them longer than the games they won. That's because the emotional sting of loss is greater than the emotional charge of gain. Negative events or "hits" affect us more strongly than the relatively positive. (For more see Bad is Stronger than Good)
I strongly believe the Scriptures teach us to monitor our thoughts (Phil 4:8,Proverbs 4:23). It's imperative that we set our minds on God's reality over against our own reality. Without understanding what God's reality and freedom is we're all predisposed toward fixating on our mistakes, problems and our past.
However, that doesn't mean that all negative events or moods are destructive. Not only can critical or cynical dispositions save us in times of danger they can make us more effective at certain tasks. For example, I've learned after reading Emotional Intelligence that balancing my checkbook or putting a budget together while in a bad mood actually orients me to be more realistic about how I'm spending my money. The trick is getting myself in a bad mood!
To take this all a step further... I don't believe the the goal of the believer is to be free from emotions. I'm not even convinced that we can be free from their influence. The best assessment I have right now is that the goal is to be from the control of emotions. 2Corinthians 10:5 exhorts us to take every thought captive, submitting it to the authority of Christ.
So be aware of what you're feeling. It is a part of how God made you. But don't be driven by every emotion you have. Don't believe everything you think.