At first I was gonna quote myself with one of those 'there's nothing wrong with liking your art' posts, but honestly this addition puts a different spin on the situation...
Although I do think I have progressed overall since my earlier comic days, I also feel kind of envious of my past self, namely with backgrounds, as I've mentioned before. ^^; The scenes I used to be able to draw (with nothing more than a pencil!); the patience I used to have before academics shorted out my brain...it's hard not to feel like I've progressed in reverse in some areas.
And of course, the better you get, the more you plateau...I can't really evaluate your skill, but that could be a reason why you feel stagnant. At some point, you're just doing what you do and there are no surprises left. In order to keep progressing, you have to go out of your way to challenge yourself.
That's basically what I've done throughout my life with all the many art styles I've learned...after I have one down (I usually don't actually have it 'down' yet, but I feel that way, at least) I get kinda bored with it, so I look for another new skillset to learn.
And eventually all the learning melds together and makes you better in general: I tried realism for a while; it made me better at manga. I copied a how-to-draw-Pokemon book once, and although the Pokemon drawings I made were pretty bleh; trying to emulate the coloring in the book gave me a better understanding of shading and lighting. Every little bit helps; even when it seems irrelevant. ^^