doggedly

I grew moody, silent, and unsocial, but studied on doggedly and incessantly.

'Nay,' said Dr Johnson, 'a man may write at any time, if he will set himself DOGGEDLY to it.' [Footnote: This word is commonly used to signify sullenly, gloomily and in that sense alone it appears in Dr Johnson's Dictionary. I suppose he meant by it 'with an OBSTINATE RESOLUTION, similar to that of a sullen man'.]