Speaking a sentence should be like throwing a ball.
In the sport of baseball, you are taught to throw the ball so that it gets into the mitt of the recipient – so that they can catch the ball securely. In most scenarios, that requires you to throw the ball with considerable force. Distance is a big factor, but even when the target is somewhat close, the ball is easier to catch if it has some force behind it.
In speaking, the sentence is the ball, and the ears and eyes of the listener are the mitt. As the speaker, you are responsible for speaking each sentence so that it gets to the ears and eyes of the listener with some force – so that they can catch the sentence securely and process it. Common less desirable speaking habits include volume trailing off at the end of sentences, a frying-like sound to your voice (vocal fry), and a consistent upward vocal inflection (up-talking).
Watch a recording of yourself speaking. Are your sentences easy to catch securely? Or do you have a habit of throwing dribblers?
Focus on follow-through.
In throwing a baseball, follow-through involves completing the entire throwing motion by moving the hand through the target even after the release of the ball. The speaking equivalent is maintaining the force of your breath all the way through to the period. The period is a definitive downward sound on the last word of the sentence.
To follow through, you must not only maintain your force of breath through to the period, but you must also maintain your force of intention to get the thought into your listener's mind...even past the period and into the split-second of silence that follows.
To improve your follow-through, practice by talking to a framed portrait while recording your voice with your phone. To the person in the portrait say, "I really believe in this. This is something I have to do." Put the same intention into transferring each sentence into the mind of "your listener" as you would if you were throwing them a ball. Follow through.
Listen to the recordings. After you are able to maintain your vocal force through the period, try two other sentences of your own choosing. Then three. And so on.