Am able to speak decent english. but have been a shy and nervous person throught my life. am 20 now. am aloof ?

am reaping the consequences of this bad attribute of mine now. am not able to speak boldly and i make grammatical mistakes due to fear. my legs start trembling when i speak to the interviewer. its simply embarrasing. got rejected by three call centre in the past week. they say am nervous, have no confidence, not enthusiastic and have zero energy level. am a slow learner also. i dont feel easy when people are around me. i feel ashamed. cant speak boldly despite being able to speak good english. i have this negative quality also. before undertaking any work i feel that i will not be able to do it and what will other people think of me if i flunk. this is the 1st thing that comes to my mind. please give me some tips to speak boldly and confidently. its high time i change my attitude in life.