(it is hard to see all your values, it could help to post each step in your work)
ok,,, for now use 1/rt = 1/r1 + 1/r2.... to calc your parallel resistors just to keep in a trend for now,
first you must note there are NO independant resistors there,,, as i go through the steps redraw your circuit each time to follow.//
our object here is to simplify, to do this we will first compress all parallel branches, as ron says do R1 and R2 first(parallel calc) dont forget ..... then add R4 and R5(parallel again) ......... then do R6,7,8 ........
NOW it is cleaner and we can add the series resistors,, so it s simple adding(for simplicity the answer you got for R4parallelR5 will be called R4,5 K?)..... so first we take R4,5 and R9 and add them(series), then we take R2,3 and add it with R6,7,8 AND with R1 aswell(series),, now adding R9 and R11 together will also simplify that branch.
lets pause here since you are wondering about adding R10 to that series part , and the reason we couldn't do that is cause R10 has a parallel branch with R9,11 but now since we have added R9,11 we can now take that answer and simplify the parallel with R10 , once you have that branch simplified THEN you can add it to the HUGE SERIES part we just did,,,
simplify...... at the end you will get a resistor(equivalent) in parallel with R12......
!!!!!