Hello there (:
I scrolled through your blog a bit.
You've made some nice progress since the start of your sketchbook~
The rendering in your mermaid piece is really lovely btw; very crisp!
Regarding the finished feeling, something that could help you out is taking works from artists that you really admire. Then, you can take a finished piece such as your mermaid collab and compare that with the piece(s) of the chosen artist(s). Analyze and jot down notes of the differences in levels of finish between the pieces. Think about what the artist(s) did and how you can possibly apply that in your future work.
I hope that helps, and if not, feel free to shoot me a message for an overpaint!