I believe the consisteadncy is an important thing in learning English.

"slow and steady" is also a phrase you could use here, but I think consistency is closer to your intended meaning.

I will try to write senetences every day even it isf they are very short!

I am really eager to beingcome fluent in English...

I believe the steadybeing regular/consistent is important thing in learning English.

I am really eager to being fluent in English..

I believe theat being steady is the important thing in learning English.

being steady/being consistent

I think I'd say "when learning"

I will try to write some senetences every day, even itf what I write is very short!

I'm really eager to being fluent in English..

or "I look forward to being fluent in English".