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i have this problem too! what i do is make multiple layers of sketches of the same main sketch while referencing poses / anatomy to get a clearer picture. if i stray too long i go back to the main sketch and look at what i'm trying to portray + the angle & pose https://twitter.com/mikumo112/status/1305878973423841281
don't jump into lineart right away unless you're confident in your ability to translate your knowledge of anatomy to lineart (i know i can but i can't execute it without some kind of reference)
once you're happy with the idea of your piece, add in features that will further your concept. the basic pose (see below, then the 2nd) are still messy and that's okay! touching up takes time but the important part is not to lose the "essence" of your first idea.
(it's a messy doodle to me) but you get the guise. hopefully this helps to all you out there! don't worry about making your first few sketches super messy - there's always a time and place to clean it up afterwards.
and if you still feel frustrated, take a break and come back to it with a fresh mind. i guarantee that there are angles you haven't seen with your art that weren't there before.
