It becomes difficult to translate thoughts from mother tongue into a language that you are not used to speaking. The best advice I can give you is to begin thinking in English. When you think in English your thoughts and words will sync automatically.
The next thing you can do is practice speaking on random topics, on your own. Stand in front of the mirror and observe your body language, as you speak. Also, make note of what happens when words don't flow – is it that you have nothing new to add or is it that you are failing to put your thoughts into words.
You don't have stage fright, which is a good thing. Whenever you have a speech to make, make a mental note of points that you have to speak on. Better still jot down key points on a piece of paper and use them for reference. Keep going over the key points over and over again, before you hit the stage. This way the points will stick in your head and you'll be able to talk about them.
If all this doesn't work – go back to what you said before you got stuck and elaborate on the point. You forgot what to say, or words don't flow, you go ahhh, and ahhh again. Don't lose confidence. Instead tell your audience, before I go any further, I want to elaborate on the point I just made. Talk on that subject some more and take it from there.
Also, never memorise a speech. You are doomed if you forget even a word. Always work with points and learn to talk on those points. Also, construct short sentences, rather than long ones. Keep your speech short and concise. Always start with an anecdote and end with a punch line. Have a few witty things to say.
Learn to improvise by bringing in new elements into the topic. Connect points - for example, pick up something that you spoke about or something that was part of the discussion and bring that into the speech, while keeping it relevant. Learn to think on your feet and react.
Underestimate me...that'll be fun!