I am a self taught digital artist (pop-culture / sci-fi genre) who can't seem to get over certain humps. I can get the broad strokes looking OK but always stall out in an unfinished state (pic rel)...when I get to this stage where I need to tighten up the loose strokes and "finish" the image...well it falls apart.

For instance, for those who know Matt Dixon, I have done his tutorials and can get to about his 70-80% mark...and then he just goes in and tightens it up...wham, bam...and his pieces are completely finished and professional looking (2/2 below). Mine look tight and shitty when I try.

So given this somewhat vague rundown...and words of advice or tutorials / professionals you could recommend studying? TIA