I have a stiff tongue, how can I cure it?

I used to be able to speak extremely fluently, and I was good at making speeches.
I could articulate words very well.

But later, my tongue got more and more "stiff", cumbersome and awkward as time goes on.
I found it much harder to control it compared to the past. Its like, I know exactly what I want to pronounce, and I know exactly how I am going to pronounce, but I just sound awkward when I speak,
its like my tongue preventing me from speaking the way I want, cos its not moving properly.

Does anyone know the reason to this? Thanks!