I have found out that rep number and not weight is my enemy mentally. I mean yeah, when the weight gets really heavy (3RM or more weight) then I'm mentally challenged to do that rep, but in a 5RM environment I don't have a problem. I have a problem with more reps though, 8 reps is a lot with a heavy weight for me.
That's why I'm staying in the 8RM realm for a while - to build that mental fortitude. And I know 8 reps is not much by fitness standards, people are squatting 20 reps etc, but for me it's borderline to "very many reps". Probably because I have trained so much in the 5RM thing.
Well anyway, the point was that yeah, at least for me, you need to have that "mental focus" during 8 rep sets to keep on using perfect form and to keep on going.