long time no see /ic/, i am having a problem that i dont think google or youtube can solve.

ive been doing some independent newbie work for a while now, just picking up jobs as i could get them, and i recently took on a piece that was in my wheelhouse and seemed like something i could do. im a bit rusty as i was struggling to find time to draw before i took this job, but i didnt think it was a big deal. ive had this painting sitting in my lap now for a while and i just cant do the fucking thing. my brain was melting apart trying to fix characters into perspective and it started ruining my thought process, now everything i draw looks like shit and i feel worse than when i first started learning years ago. faces i draw look wrong, poses/gestures look wrong, composition, everything. i get so frustrated trying to tackle this that i want to tear my fingers off like an enraged silverback gorilla.
normally id give up but seeing as this is work im getting paid for i have to finish it, what can i do to break through this neuroticism? any exercises i can try? lectures to listen to? should i try to mock the drawing up as best as i can in 3d and just work over top of it? im at a loss and im panicking

i would post my work here but seeing as this is for a client i cant really share it