You mention you have been doing it 6 months. Are you still progressing or have you stalled? When did you last add weight or reps to each lift? You mention weak core, are you doing anything specific for core ? If squating 3 times a week is a struggle have you tried 2 days a week?
Maybe give us a bit more detail and be more specific with your questions.
Well, I was starting to stall... each workout was a sweaty, full-on, epic struggle to push out 5 x 5 on the weights I was doing (I was fatiguing mentally)... so I tried to change it up, I tried:
1) higher volume, lower weight, 25+
2) different reps/sets, eg, 3 x 8
3) going to failure
Then I started getting injuries... first my lower back went. I was doing deadlifts, it felt strange... then the next day it was sore and achy. I saw a PT, he said my legs were too long and my hamstrings too tight and recommended going to sumo deadlifts... but I had same problems with sumo, when I went heavy my back started having problems.
Then I did a workout, first workout of the week, I was fresh and ready to go, I did 4 x 5 sets of 100kg squat then 1 x 8 set on 90kg squat to failure (I felt strong)... then the next day my knee was sore, it was a muscle soreness, not a joint... I think it must have been a muscle strain. I couldn't even squat the barbell 2 days later on my usual wednesday workout.
Then on the overhead presses my right shoulder started to get sore... and wasn't recovering between workouts. But I seemed to remain strong on my overhead press and bench press.
I have had the last week off... my knee feels much better and so does my shoulder. The only thing I am concerned about is my lower back... the physio says it is really tight and has given me stretches to loosen it up.
Someone recommended on the american body building forum to do romanian deadlifts instead of normal deadlifts... I've done one workout with RDL, only 40kg, but they felt good, I was getting a good stretch in my hamstrings.