Couple of thoughts.
At 5'10" and 185, your a pretty thin guy, which will ultimately affect your gains. You need to gain weight for this program to really work, and I don't mean be a giant fat ass.
You're really dropping into the bottom of the squat, remain tight, tight, tight, and I think you would easily have made that 5th rep, it was still going up when you quit on it.
Finally, looking at that set, I'm guessing you did give up on the sets across prematurely.
What do I recommend? Drop to 285, work 3x5 until it stops. If you do that 3x a week, you'll be back at 315 in two weeks. Stay super tight. Eat more.