Take a break? I mean, don't take a such a long break that you forget what you were doing in the first place. Just like a short one, a day or two. then come back to your piece. Or search for some inspiration out there. When I'm writing, or coming up with an idea for a game I take the things around me and try and work them into it. Red and interesting book, watch and interested movie. Listen to some new and weird music. Hell just go for a short walk around your neighbor hood. Walking stimulates the brain. My Partner and I go walking when we want to brain storm.

My favorite thing is to take a bike ride around for an hour or so - I find it really helps me out of creative ruts for music. Not sure if that translates into visual art, but it might help. Also a simple meditation routine can work wonders on cutting out mental distractions and depressive bouts that kill the creative drive.

Try a different tactic. When the way you work stops working, work a new way. Set a timer, give yourself an hour. And do as much in that hour as you can. Race yourself. Even if it is shitty quality when you are done. You don't have to post it and show everyone your practice work, just keep it for yourself. Hell, erase it when you are done if you want. (but I would suggest keeping some examples so that you can keep track of progress).
Do it every day. Rain or shine, even when you wanna be all "f*ck art".

It will be hard. It will be onerous. It will make you grumble and bitch and wanna throw things. It will also make you faster and better. Art is as much a talent as it is a skill. And it doesn't grow without practice. Just like a marathon runner trains every day by jogging, so do artists train by pushing through.

Its hard, I know it is. That slump, that plateau when you just feel like everything should burn because it doesn't feel like its getting better and there's nowhere to go b ut off the edge. But you can do it. you can find a way.

Watch some speed painting videos, really watch them and try to figure out how they are doing what they are doing. (which is hard for me cause half the time I just stare in utter amazement and go like.. 'oooo shinnney ...') but turn off the amazement side of your brain, and the jealous side, and turn on the technical side. a lot of them use barely any sketch. they go straight to pain and then just rework the colors over and over, defining more and more with each go round until its a thing.

There is no need to actually finish art^^ For every piece I produce there is at least 5 sketches I just delete - even if they had promise. I know some artists have their own folders for them but eh.. that eill just stress me so I rather just delete everything and leave it in the past ^^;; just focus at it one step at the time. if it just ends up as line art - then no shame, unless it's a paid commission you don't really -have- to make every piece a full completion.