Hi, I have a mesh for an object that needs the mirror modifier to keep its form. However, when I try to texture paint a stencil overtop, the stencil mirrors over the object as well. This is problematic, because the stencil itself is asymmetrical. I have no idea how to unwrap the UV map so it will not mirror. Any advice on this will be super helpful, thank you!
Welcome 
,
if you wanna have an unsymetrical texture over an symetrical object you have to apply the mirror modifier.
You may separate the object in symetrical and (texture based) asymetrical parts and just apply it to one.
Hi, check “mirror U&V” under the data panel in the mirror modifier
Ups, yes indeed… Thanks, i learned something new. (Or rediscovered this…?? Sometimes i think i knew something from 2.79 but had overseen it since the changes to 2.8…3) …hmmm on a first look it seems a little weird to check both mirror UV to not mirror it…
This is pretty new iirc
Ups, yes indeed (again 
) … this was just U/V offset and Flip U/V…so thanks again i learnt something new new… (and because of this i will remind better).
Hey! Thanks! this advice helped. As it turned out, just the applying the mirror modifier was not enough. I also had to re-do the smart UV map projection. After that there was still some weird warps and reflections here and there, but I think that is because my typology was super messy XD
Thank you everyone for the help!