If it doesn't feel unnatural, you aren't trying hard enough.
I still slouch a little, but I used to slouch more. I didn't really care about things like posture when I was in my teens and early twenties, but my mom did. She was the slouch-cop, often scolding me to stand up straight and pull my shoulders back. It felt unnatural and tiring to do so. It took effort and concentration. I can't really say when I changed my posture habit but I did. Now standing up straight and pulling my shoulders back feels natural and fairly effortless.
A lot of us have slouch-like speaking habits. We might fragment sentences, ramble, drone, mumble, quiet-talk, up-talk or trail-off. Those of you who have worked with me know that I'll help you raise your awareness of such habits. I'm sort of a slouch-cop of speaking. More importantly, I'll show you how to "stand up straight and pull your shoulders back" as a speaker.
Here's the catch: it's going to take effort and concentration. It may feel tiring. And it's going to feel unnatural. Doing anything outside of your existing speaking habit-pattern will feel like you are not being you, but don't be confused.
Speaking expressively in clear, concise sentences with relaxed power and articulation is natural. It just may not be your habit.
The behaviors behind impactful speaking are really not that complicated, but our individual habit-patterns continuously evolve and are prone to little defects. If your goal is continuing improvement as a speaker, you should routinely feel that unnatural feeling associated with habit change. To an extent, you should seek it out.