I think the main reason is lack of practice. You need to just practice speaking. Find a language partner that wants to study your language and you do an exchange: you help them learn your language, and they help you improve your English speaking skills. Go to this page, under "he is learning" select your native language, and you can find some possible partners you can contact: http://www.italki.com/partners#learn=all&speak=all&page=1 Also when you study on your own get used to pronouncing things aloud. For example practice reading aloud, by finding an audio-book (a book that also has a CD of someone reading it). Try reading and then listen and repeat to correct your reading. This not only will improve your reading skills and your pronunciation, but also it will loosen your tongue to feel less afraid to speak that language with people. Have fun!! Language study is fun.

Lydia's right - you need practice. This means opening your mouth and using your voice. This means physical activity. Think of a person who can read music and who listens to music often, but never touches a piano or picks up a violin. In spite of their knowledge, they play badly. The brain is familiar with music, but the fingers have had no practice.