My first diagnostic was a 139. I studied for 5 weeks, took the actual LSAT, and got 155. I'm studying now to retake and my best so far has been 166. Hang in there!
Yeah, that mindset that you will lose points no matter what isn't good. I know someone who had never gotten higher than a 167 on a PT and got a 171 on test day.
Just go in with a positive mindset and do your best!