First, remember that you are not your job. That is, if you make a mistake at work, this does not mean that you are stupid, worthless, or that you're in the wrong position. It's all too easy to take mistakes personally, seeing them as a reflection of your true person rather than for what it is: a mistake.
Even though it may not always appear so, everyone makes mistakes from time to time. The best way to deal with a mistake is to own up to it right away and present a solution. This shows that you are honest, and by presenting ways to fix the problem, your boss can send you on your way to deal with the issue. Acting honestly and straightforwardly is best for you--you'll feel better about yourself--and best for the company (which again will help you feel better).
Another common issue is feeling insecure when it comes to coworkers. Many people feel that they do not fit in, are unsure how to handle conflict, or have an overbearing coworker or boss that they don't know how to communicate with. Any of these feelings can wear at your self-esteem.
You may feel you have nothing to offer the group, whether socially or on projects, you avoid conflict, and may allow others to step on you. If socialization is a problem, it will require you to step out of your comfort zone a bit. This does not mean you need to jump right in with a large company gathering; rather, take it slow by opening conversations with one or two coworkers.
