I am having a very serious problem regarding my SPEAKING SKILL. The Problem is that it is very hard to create a good sentence automatically when being asked suddenly out of no where without thingking before. Its like all the words disappear from my mind when I have to respond to someone's question or comment. I couldn't find the appropriate/proportional word. And when I couldn't find the right word, I tend to create a very ugly kind of sentence consists of so much unproportional word, messy grammar and mistakes in pronunciation.
While writing, like writing this thread, the flow of words are very strong. But its very different from the speaking. I always use the same word over and over every time I try to speak. I feel that being creative by using different kind of word is very hard in speaking.

Anyone of you can help me solving my problem. I really want to speak english really well. The words are flowing and the sentences are beautifull, far from repetitive world.