There is a whole school of contemporary psychology called "cognitive psychology." Cognitive psychologists argue that we can control or diminish destructive emotions (such as the ones you've mentioned) if we can alter our thinking by being as reasonable as possible. A really excellent book on this subject is titled Feeling Good, by David Burns. I often recommend it to students; you should easily be able to find used copies, since the book has proven so popular over the years. Burns will give you not only much good advice but also many practical exercises to do that will help you with the problems you mention.
Being shy certainly causes stress--is this the implication here?
Psychologists have long believed that it is the parents who instill confidence in their children, so if one is shy, often the problem is the lack of approval from loved ones. Nonetheless, there are many in our present society who have PhDs in self-confidence with very little accomplishment and talent behind it. However, there are still some who are not objective about their talents in the other direction--they are too self-effacing.
From experience more than anything else, a person becomes more confident. Entering some type of competitive activity and winning does much to overcome one's shyness. Perhaps, one can begin with activities in which one feels the he/she can succeed; later, the person can venture into other areas. Often, if one enters these activities with a friend as support, the stress of bravely putting oneself forth is mitigated. For instance, double dating helps many a shy person since the friend will talk and include the shy person automatically. The old axiom, "Success breeds success" is absolutely true.
Those are two very different things, aren't they? To fight shyness, I think that your only real option is to confront it head on. I think that a shy person has to put themselves in situations where they feel uncomfortable (probably a little at a time) so they can gradually get used to just dealing with acting in more outgoing ways.