well we can do this the tough way, by going way back to the basics, life drawing, trying out new techniques, cold turkeying anything that is comfortable to you.

Or.. you could ease into it where you take one day a week to do study of anything , to draw it or paint it to make it the best of what it is.

ooorr..slow mo it, keep doing what you do, and draw whatever takes your fancy and watch some videos of others painting away.

Your cel shading looks really good. I'd like to see more of those hard edges used in your more realistic shading styles - 'soft shading' as it were - to really push the realism. True shadows have both hard and soft edges. I'd recommend reading up on edge use in general to try to get a good handle on that.

Life drawing is definitely essential. You've simplified so much for so long that your anatomy, like your soft shading, has gotten kind of smooshy and lacking in definition. I'd recommend trying out drawing some more muscular characters to get a good idea of what goes where.

Lastly, use reference! All the time! For everything! It's the best way to corral your style back into something you want it to be. Make sure you're altering reality on /purpose/, not because you can't recall what a bicycle looks like offhand, you know?

Some resources:
http://artists.pixelovely.com/practice-tools/figure-drawing/ (life drawing)
http://ctrl-paint.com/ (general advice)
http://www.huevaluechroma.com/ (modern color theory)
http://anatomicalart.tumblr.com/ (general anatomy references)
http://www.conceptart.org/forums/art-tutorials-art-tips-art-tricks-archive/51913-rendering-discussion-edges-tips-tricks.html (edge control)

Aww hun :<
If it makes you feel any better, I can honestly say that I've gone through this before too. Mostly because I saw all these wonderful, cute drawings by amazing artists and I became sort of jealous and envious of them because well.. my art has never been the defination of "cute" and even when I'd attempt to draw cutesy things, it would always get shot down because there'd always be something weird or scary about it. So for the longest time I would just avoid drawing cute things and it wasn't until recently I learned that I should stop comparing myself to other artists, and draw what I want how I want because it's /my/ art. I can sorta draw cutesy things now, but they're mostly just dumb doodle things that I don't share online xD

So speaking from experience, I know how tiresome and frustrating something like a change in art style can be. I know I've dabbled into a LOT of different styles throughout the years but my current style just kinda developed on it's own and stuck. But even so, I know it will change again in time.
To me, art is like watching the leaves change with the seasons. It grows, it blooms for a bit, it falls away, and regrows again. Art grows with us throughout the years- sometimes it's super obvious, sometimes not so much. But it never truely stays the same.

In regards to your situation, I agree with Happysorry, it might be good to go back to the basics and practice, or at least warm up a bit. But be sure to do whatever is comfortable for you, since this is art on a personal level you shouldn't have to drag your face across pavement to get there. Let it come to you slowly and naturally. Doodle your characters, use them to help you practice coloring/shading techniques and anatomy. Draw things you love, like food/items, backgrounds, ect. Maybe even just shapes!
My time spent studying art for my college classes online taught me that whenever I want to try something new or practice, I can always go back to basic shapes like circles and squares. I used to hate that because it felt too much like 'homework' but now I've come to enjoy it a lot. Especially for shading and coloring practice.

And of course, look for what inspires you. I have a folder on my desktop that is filled to the brim with inspirational images and gifs. Mostly referrence images/gifs, but I also have concept art from games/movies, along with some photos with inspiring color palettes, since I want to expand my color usage and studying colors inspires me so much.

So making an inspirational collection may help too.

Wow this is getting long oops x.x;
so tldr; don't compare yourself to others! You are YOU and your art is YOURS so be yourself <3 Take your time and sketch, doodle, and practice- let the progress come to you and be patient, find what inspires you and learn from it.
Art is a gift many people don't have, it shouldn't feel like a curse. Do what makes you happy and grow from it <3

I can not ever recommend life drawing enough. I took a class at my college and made a huge improvement with my work in that semester alone. Observational skills and a good understanding of depth and perspective are very necessary tools for an artist to have, and the only real way to develop those is to draw what you see and draw it often.

Gestures. Draw a lot of gestures. Loosen up your sketches before you worry about details. Be sure to draw through the form rather than around it. This allows you to recognize key shapes and lines, which makes for much more interesting movement in your work.

It seems like you've been observing models. If you've got the nerve for it and there's availability in your area, maybe try modeling for a drawing class as well, or even just a trusted friend. Some classes will only accept nude models, but some will allow models to wear their underwear. Holding those poses gives you good time to think about how your body moves, where your weight is shifting and resting at, and tension builds in certain areas when you stretch.

Daily sketchbooks are also a great way to practice speed and focus - make yourself draw one scene for 10 minutes non-stop every day. Spend about one minute on the basics and nine on details.

Don't ever stop drawing things you enjoy though, or you'll get bored and frustrated. Challenge yourself and reward yourself for meeting those challenges.

Personally I love your art and I'm not in any place to critique it in the slightest. o.o
I hope you're feeling alot better in any event.

LATE COMMENT, but I don't think there is anything necessarily wrong with simplifying your style and going back to the high detail. It is good to build up that versatility. It might seem frustrating but the fact that you even produced art is a great thing. Going back to the style you used to have will probably be hard only in that you have improved SOMEWHERE (such as in condensing detail) and it is hard to mentally let that go. BUT IT IS DOA-ABLE. If you must, pit yourself around people who actually DO make that kind of detail you want back in your work. Buy some artbooks and study other styles. Everyone has already suggested references, but there are other things to it like garnering that inspiration.

I know how this feels and I find myself envying many a pixiv artists. Im always pushing myself to be detail forward, but also have days where I just want a simple and cute look (see:lazy). Do not lose hope on yourself! You can do it!