I am almost the exact same height and weight stats and had the same problem for a long time. After trial and error and alot of questions I found out:
1. I wasn't eating the right foods before and after my workouts
2. I wasn't getting enough sleep
3. Take a week off every 2 months or so. This helped my body really recover, I noticed an increase in desire, recovery, explosiveness, and overal energy which got me past alot of the mental blocks that are present when maxing out.
4. Do high reps for more then just one week at a time, right now I do 10 reps on week 1 and 2 , then 8reps for 2 weeks then move down to 6 reps. This helps me mentally get the 6 easier and by the second week my max is usually a little higher.
I hope these ideas help a little man, most of the problem is probably in your head. Remember GO BIG OR GO HOME.