I was in the flash chat thingy, and it was bombarded with infp's talking about their regrets. I lectured to them about how regret is a useless emotion.
Anyhow, I consider any negative emotion such as feeling worthless, sad, depressed, etc to be useless. They accomplish nothing worthwhile. On the other end of the spectrum, emotions such as happiness, satisfaction, cheerfulness, driven i see as useful emotions since you feel good and you set yourself up for personal growth.
I've trained my brain to think positively about life. Of course this comes to me naturally as well, as I am an optimist. But when I was a teenager I thought negatively about myself at times. I realized though that thinking negatively about oneself makes you feel "down". So, now if a negative thought pops into my head I challenge it and see if it's logical. Very rarely is the thought actually logical. And if it is, that means there is something about myself that I need to change, and then I change. And BAM that thought never has to come back.