how could I say?
I became still more comfortable with Speaking English.
I became still more comfortable in Speaking English.

Question

Answer

I think you are trying to say:
I have become even more comfortable speaking English. 
Hopefully I am interpreting the thought you are trying to convey correctly.
Hope it helps :)

Answer

You could also say...

I am still trying to become more comfortable with speaking English.

Answer

Not easy to decipher but let me contribute with my version:

"I have become even more comfortable speaking in the English language"