I have been busy these days, so I haven't been studying English at all. Yet I hope I can still find time to study a little bit, however busy I may be. The construction is "I haven't been ADJECTIVE". So when you say "I haven't been studied", you are using "studied" as an adjective to describe yourself. That is a good sentence, but it means that no one has studied YOU. That is clearly not what you intend to say. "Studying" is also an adjective, but it does describe you.

I was busy these days so I haven't studied English at all. I hope I will still find time to study a little bit however busy I am.

I was busy these days so I haven't been studying English at all. I hope I will still find time to study a little bit no matter how busy I am.