We like to think that we use our thoughts to figure things out, analyze problems, plan ahead and make decisions. But we also use our thoughts to avoid doing uncomfortable things, escape from stressful situations, protect ourselves, justify ourselves, make excuses, avoid guilt, etc., etc., etc.
For example, if we're more comfortable with being rejected than with rejecting, we may think that a lover wants to dump us when, in fact, we want to dump him or her.
Or, if we're not comfortable with being angry, we'll think that somebody is angry at us when, in fact, we're angry at them.
Or, if we're not careful, we'll use a thought like "Oh, this will never work" to avoid putting all of our effort into a difficult task the success of which is uncertain.
Negative thinking can make us depressed. Some of us pay much more attention to negatives than to positives. Or we "catastrophize" minor setbacks into major ones. Or we make a sane statement to ourselves like "I'm really depressed over my break-up with _________" and then follow it up with an insane statement like "I'll never find anyone to love again. I might as well give up."
Sometimes this negative thinking becomes automatic and takes on a life of its own.
Al Galves is a psychologist who can teach you how to become aware of how you are using your thoughts, learn about yourself from that awareness, and learn how to manage your thoughts so that they help you instead of hurt you. Through reading his book Lighten Up: Dance With Your Dark Side and attending his workshops, you can develop the skills needed to take more control over your thought process.