Hi Haru, yes that is a common use of the phrase - ‘I don’t find it easy to concentrate’ , ‘I don’t find it easy to get up in the morning’ etc. It’s a less direct way of saying ‘I find it difficult to...’, in UK English indirect speech of this kind is quite common.

Hello Haru, Usually "find something easy" is used in the affirmative. Technically speaking, you could use it in the negative (in speaking), but I haven't heard it used often that way, at least not in American/Canadian English. British English may be different as indicated by Tom in his answer. More commonly, "find something difficult" is used to express difficulty with something. Hope this answer is useful to you.

Perhaps because you've heard other people say "I find it hard/difficult to concentrate.

Perfectly natural and a collocation used frequently. In the positive voice.

Yes, it sounds perfectly natural.