I think I only use the expressions I'm used to.

To be a fluent English speaker, I think I should absorb new expressions and use them frequently.

But since I have reached the basic communication level, I always use the expressions I'm used to.

I have to overcome it. I have to learn new expressions continously.

...

I have to overcome it. I have to learn new expressions continually.

Tricky: continuously is without intervals of interruption. 

Continually is for a certain duration, with interruptions.

Not to worry, everyone will understand both when in context.

I think I've just written I'm just using the expressions I'm used to.

To be a fluent English speaker, I think I should absorb new expressions and use them frequently.

But after I reached to the basic communication level, I always use the expressions I'm used to.

I have to overcome it. I have to learn new expressions continously.