Dear friend,
This is in addition to what Mr V. Raghunathan has said. Practice makes man perfect goes the adage. Therefore, what you need to do is to start talking in English with your friends. Develop friendship with those whose mother tongue is different from yours.
Your problem why you are unable to speak in English is probably because you could be thinking in your mother tongue. Writing gives us permission to think in mother tongue and then translate. Furthermore, while typing on computer or laptop, making corrections is so easy. Speaking offers no such privilege because of lack of time gap. Hence your problem. Therefore, start thinking in English as well.
The silver lining of your problem is that you have identified the problem and you have approached this forum for the solution. People do not even understand what their problems is. Yours is not this case therefore, I am confident that surely you will tide over this challenge.
Take the case of our Prime Minister, Mr Narendra Modi. Before becoming PM, hardly he spoke in English. Just six months in power and he has changed himself and he has started giving speeches in English. His position is so high that it offers no room for experimentation and he has to speak correctly per force. But then he has been doing it. Take inspiration from him and go ahead!
All the best!
Dinesh Divekar