It just takes persistence. Try to speak with as many people as possible, until you feel entirely comfortable expressing yourself in English. I know quite a lot of people who think they are great at English, but they still make plenty of mistakes.
Hehe, don't let this discourage you. A man who knows his weaknesses holds the key to success. As long as you're not complacent, you're going to be just fine. Hehe, and that key to success is practice, because remember: "Practice makes perfect."