oh yeah i totally get that feeling.. perhaps think back to why it is you want to draw that thing.. if there is nothing making u excited about it then there might not be enough motivation to keep u going.
for example i used to hate drawing buildings cus they never looked as good as i imagined them to be.. but because my goal is to turn my story into a comic - comics need backgrounds - ie buildings i was like i gots to do it.
i started with - what is it that i really wanna see, perhaps a specific scene in my story im really excited about.. and i'd think about the finalized pic and how it would look. usually the "i dunno how to draw that" usually comes from having an idea of what i want but not thinking about the details in my head. usually after thinking about how that finished piece would look for a few days to a week or so and have a solid idea (if im still iffy i grab some refs) it usually ends up so much better than before i started to think in DETAIL about it.
hope that helps 
and sometimes its ok to chill - i totally am guilty of pushing myself for my project but sometimes i get burned out and just wanna chill lol and i try to avoid burn out by not pushing myself too much and take breaks if i think im getting close to a burn out since if i spend a day or 2 taking a break it usually means i won't burn out for 3x the time ie 1-2 weeks of burnout break.
i recently just finished my mv and am feeling a bit lazy so a part of me feels bad about it but at the same time i think i should take a break before moving on just in case since that mv took 200+ hrs over the course of 2 months where i only took a day or 2 of breaks in between.
