Imagine you have a special word, like “happy” or “big”, or even “interesting”, and you have to use it all the time. Won’t you sound boring? Maybe awkward? Sure, you will. But guess what? There are so many words you can use instead of “happy”, “big”, or “interesting”, and you have to learn these words in order to improve your speaking skills, and to impress your interlocutors.
When you use different synonyms of the same word in your sentences correctly, you show others your high level in English. The reason why many students think their English is bad is because they repeat the same words again and again, because their vocabulary is poor.
To speak more confidently, you really need to enrich your vocabulary so that you can engage in interesting conversations, vary your expressions while speaking, and get your message through more accurately and appropriately.