I think part of it is a form of stage freight from that shyness. I have this problem a lot too when I"m trying to communicate in whatever language I'm learning. What's helped me is finding someone that I feel comfortable making those mistakes with, and forcing myself to find whatever words I need and speak. It can be really hard, but it's worth it.

I had the same problem at the beginning - I was very shy and blocked. The only thing that helped, and it may sound like a paradox, was talking to people. I had to break through my limitations and fear of not knowing/not understanding/making mistakes. The only thing, then, that can help is conversational practice. So, go out there, and just talk to people. The words will come to your mind easier and faster eventually. And soon you will notice that you're already thinking in English. You have the vocabulary in your head, you know it. It is just a matter of not stressing so much. Believe me, it may be terrifying at the beginning, but then, all of a sudden, it will become so easy for you. You will laugh afterwards! :) Good luck. :)