Why can't I ever form long sentences when I talk and speak?

You're letting the stress of what people are thinking of you get in the way. This stress is blocking your ability to express yourself in a meaningful way. You may want to contemplate on what I have just said because thoughtful contemplation is simply being truthful with yourself. Another method you might try is insight meditation. There are many books on the subject and I would select one that is simple and gives basic instructions for beginners.. Meditation is an excellent practice to slow down your thoughts and develop an awareness of what is going on in your mind from moment to moment.